Can't build with LLVM support

Build steps:
```sh
$ git clone --recursive https://github.com/google/autofdo.git
$ cd autofdo
$ git checkout 0.18 # missing "perf_data.pb.h" on master
$ aclocal -I .; autoheader; autoconf; automake --add-missing -c
$ ./configure --with-llvm=/bin/llvm-config
checking whether we can compile and link with llvm(ProfileData)... no
configure: Using static LLVM libraries.
configure: WARNING: could not detect the LLVM libraries. Support for LLVM profiles disabled.
$ make -j$(nproc)
```
Runtime error:
```
ERROR: LLVM support was not enabled in this configuration.
Please configure and rebuild with:

$ ./configure --with-llvm=
```
LLVM config:
```sh
$ ./bin/llvm-config --libs profiledata
-lLLVMProfileData -lLLVMCore -lLLVMBinaryFormat -lLLVMSupport -lLLVMDemangle
```
Thanks for looking into it.