/usr/include leaking into conda-build with openmp runtimes enabled.

### Comment:

I'm not using the conda-forge recipes (instead basing what I'm doing on the Numba recipe) but I thought this group might have expertise to answer my question. In build.sh, I'm building clang/lld/libunwind and the openmp runtimes as below.

_cmake_config+=(-DLLVM_ENABLE_PROJECTS:STRING="clang;lld;libunwind")
_cmake_config+=(-DLLVM_ENABLE_RUNTIMES:STRING=openmp)

I'm doing this in a Centos 7 VM. The build gets into building the runtime and for some of the compilation steps I get the following errors.

```
[ 12%] Building LLVM bitcode Reduction.cpp-sm_52.bc
cd $SRC_DIR/llvm/build/runtimes/runtimes-bins/openmp/libomptarget/DeviceRTL && $SRC_DIR/llvm/build/bin/clang -S -x c++ -std=c++17 -fvisibility=hidden -O1 -mllvm -openmp-opt-disable -DSHARED_SCRATCHPAD_SIZE=512 -Xclang -emit-llvm-bc -Xclang -aux-triple -Xclang x86_64-unknown-linux-gnu -fopenmp -fopenmp-cuda-mode -Xclang -fopenmp-is-device -I$SRC_DIR/openmp/libomptarget/DeviceRTL/include -I$SRC_DIR/openmp/libomptarget/DeviceRTL/../include -I$SRC_DIR/llvm/include -I$SRC_DIR/llvm/build/include -DOMPTARGET_DEBUG=0 -Xclang -target-cpu -Xclang sm_52 -target nvptx64-nvidia-cuda -Xclang -target-feature -Xclang +ptx61 -D__CUDA_ARCH__=520 $SRC_DIR/openmp/libomptarget/DeviceRTL/src/Reduction.cpp -o Reduction.cpp-sm_52.bc
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/openmp/libomptarget/plugins/common/elf_common/elf_common.cpp:16:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/Object/Binary.h:17:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Triple.h:12:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Twine.h:12:
/home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/SmallVector.h:191:12: error: no template named 'enable_if_t' in namespace 'std'; did you mean 'enable_if'?
std::enable_if_t, T *>::value,
~~~~~^~~~~~~~~~~
enable_if
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/type_traits:1766:12: note: 'enable_if' declared here
struct enable_if
^
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/openmp/libomptarget/plugins/common/elf_common/elf_common.cpp:16:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/Object/Binary.h:17:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Triple.h:12:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Twine.h:12:
/home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/SmallVector.h:191:43: error: no template named 'remove_const_t' in namespace 'std'; did you mean 'remove_const'?
std::enable_if_t, T *>::value,
~~~~~^~~~~~~~~~~~~~
remove_const
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/type_traits:1327:12: note: 'remove_const' declared here
struct remove_const
^
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/openmp/libomptarget/plugins/common/elf_common/elf_common.cpp:16:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/Object/Binary.h:17:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Triple.h:12:
In file included from /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/Twine.h:12:
/home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/work/llvm/include/llvm/ADT/SmallVector.h:204:12: error: no template named 'enable_if_t' in namespace 'std'; did you mean 'enable_if'?
std::enable_if_t, T *>::value,
~~~~~^~~~~~~~~~~
enable_if
/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../include/c++/4.8.5/type_traits:1766:12: note: 'enable_if' declared here
struct enable_if
^
...
```

It seems to be trying to get the C++ include files from /usr/include and failing because enable_if_t is not available in the base gcc available on centos 7 (gcc 4.8.5). I don't know why it is trying to use this gcc at all since the conda environment created at the beginning has gcc 7.5 in it as follows:

```
## Package Plan ##

environment location: /home/taanders/miniconda3/envs/pyomp39/conda-bld/llvmdev_1689873850502/_build_env

The following NEW packages will be INSTALLED:

_libgcc_mutex: 0.1-main
_openmp_mutex: 5.1-1_gnu
_sysroot_linux-64_curr_repodata_hack: 3-haa98f57_10
binutils_impl_linux-64: 2.35.1-h27ae35d_9
binutils_linux-64: 2.35.1-h454624a_30
bzip2: 1.0.8-h7b6447c_0
c-ares: 1.19.0-h5eee18b_0
ca-certificates: 2023.05.30-h06a4308_0
cmake: 3.26.4-h96355d8_0
curl: 7.88.1-h5eee18b_1
expat: 2.4.9-h6a678d5_0
gcc_impl_linux-64: 7.5.0-h7105cf2_17
gcc_linux-64: 7.5.0-h8f34230_30
gdbm: 1.18-hd4cb3f1_4
gettext: 0.21.0-h39681ba_1
git: 2.40.1-pl5340h36fbf9e_1
gxx_impl_linux-64: 7.5.0-h0a5bf11_17
gxx_linux-64: 7.5.0-hffc177d_30
icu: 58.2-he6710b0_3
kernel-headers_linux-64: 3.10.0-h57e8cba_10
krb5: 1.20.1-h143b758_1
ld_impl_linux-64: 2.35.1-h7274673_9
libcurl: 7.88.1-h251f7ec_1
libedit: 3.1.20221030-h5eee18b_0
libev: 4.33-h7f8727e_1
libffi: 3.4.4-h6a678d5_0
libgcc-devel_linux-64: 7.5.0-hbbeae57_17
libgcc-ng: 11.2.0-h1234567_1
libgomp: 11.2.0-h1234567_1
libnghttp2: 1.52.0-h2d74bed_1
libssh2: 1.10.0-hdbd6064_2
libstdcxx-devel_linux-64: 7.5.0-hf0c5c8d_17
libstdcxx-ng: 11.2.0-h1234567_1
libuuid: 1.41.5-h5eee18b_0
libuv: 1.44.2-h5eee18b_0
libxml2: 2.10.3-hcbfbd50_0
lz4-c: 1.9.4-h6a678d5_0
make: 4.2.1-h1bed415_1
ncurses: 6.4-h6a678d5_0
openssl: 3.0.9-h7f8727e_0
pcre2: 10.42-hebb0a14_0
perl: 5.34.0-h5eee18b_2
pip: 23.1.2-py311h06a4308_0
python: 3.11.4-h955ad1f_0
readline: 8.2-h5eee18b_0
rhash: 1.4.3-hdbd6064_0
setuptools: 67.8.0-py311h06a4308_0
sqlite: 3.41.2-h5eee18b_0
sysroot_linux-64: 2.17-h57e8cba_10
tk: 8.6.12-h1ccaba5_0
tzdata: 2023c-h04d1e81_0
wheel: 0.38.4-py311h06a4308_0
xz: 5.4.2-h5eee18b_0
zlib: 1.2.13-h5eee18b_0
zstd: 1.5.5-hc292b87_0
```

Any suggestions on how to proceed? Thanks!

Start with build.sh and reproduce the LLVM, clang, lld, libunwind, and OpenMP runtime build in the CentOS 7 environment described. Inspect which compiler and C++ include paths are selected during the failing DeviceRTL and elf_common.cpp steps. Done means the runtime build completes without incorrectly using the CentOS 7 /usr/include C++ headers.

Written by the indexing model from the issue text.