Preprocessor macros GTEST_IS_THREADSAFE and GTEST_HAS_PTHREAD do not work as expected

**Describe the bug**

At `./googletest/README.md` (not at `./README.md`) you find:

>### Multi-threaded Tests
>
>GoogleTest is thread-safe where the pthread library is available. After `#include "gtest/gtest.h"`, you can check the`GTEST_IS_THREADSAFE` macro to see whether this is the case (yes if the macro is `#defined` to 1, no if it's undefined.).
>
>If GoogleTest doesn't correctly detect whether pthread is available in your environment, you can force it with
>
> -DGTEST_HAS_PTHREAD=1
>
>or
>
> -DGTEST_HAS_PTHREAD=0

GTEST_IS_THREADSAFE always compiles to "yes" (macro is defined) no matter if PThreads are available or not.

cmake option -DGTEST_HAS_PTHREAD has no effect no matter if set to 0 or 1.

I expect that both macros work as documented.

**Steps to reproduce the bug**

Check GTEST_IS_THREADSAFE on every compiler that supports preprocessor macros, for example with GCC:

#ifdef GTEST_IS_THREADSAFE
#warning "pthread is available"
#else
#warning "pthread is NOT available"
#endif

always compiles to

/home/ingo/devel/googletest/test_simple.cpp:18:2: warning: #warning "pthread is available" [-Wcpp]
18 | #warning "pthread is available"
| ^~~~~~~

With GTEST_HAS_PTHREAD try to configure:

~$ cmake -S . -B build -DGTEST_HAS_PTHREAD=0|1

Alternative you may compare configuring with `-D gtest_disable_pthreads=ON|OFF`

**Does the bug persist in the most recent commit?**

Yes.

**What operating system and version are you using?**

Any operating system that supports a compiler that can expand preprocessor macros.

**What compiler and version are you using?**

~$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/10/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
Configured with: ../src/configure -v --with-pkgversion='Debian 10.2.1-6' --with-bugurl=file:///usr/share/doc/gcc-10/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-10 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-10-Km9U7s/gcc-10-10.2.1/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-10-Km9U7s/gcc-10-10.2.1/debian/tmp-gcn/usr,hsa --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-mutex
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.2.1 20210110 (Debian 10.2.1-6)

**What build system are you using?**

~$ cmake --version
cmake version 3.18.4

CMake suite maintained and supported by Kitware (kitware.com/cmake).

Start with the macro behavior described in googletest/README.md and reproduce it using the provided preprocessor check. Trace how GTEST_IS_THREADSAFE and GTEST_HAS_PTHREAD are configured through the CMake options gtest_disable_pthreads and -DGTEST_HAS_PTHREAD. Done means detection and both forced settings produce the documented macro results, verified on supported builds.

Written by the indexing model from the issue text.