Building and Testing LLVM libc on Windows

Setting Up Environment

To build LLVM libc on Windows, first build Clang using the following steps.

Open Command Prompt in Windows
Set TEMP and TMP to a directory. Creating this path is necessary for a successful clang build.
1. Create tmp under your preferred directory or under C:\src:
```
cd C:\src
mkdir tmp
```
2. In the start menu, search for "environment variables for your account". Set TEMP and TMP to C:\src\tmp or the corresponding path elsewhere.
Download Visual Studio Community.
Install CMake and Ninja. (Optional, included in Visual Studio).
Load the Visual Studio environment variables using this command. This is crucial as it allows you to use build tools like CMake and Ninja:
```
"C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Auxiliary\Build\vcvarsall.bat" amd64
```
Note: Rerun this command every time you open a new Command Prompt window.
If you have not used Git before, install Git for Windows. Check out the LLVM source tree from Github using:
```
git clone https://github.com/llvm/llvm-project.git
```
Ensure you have access to Clang, either by downloading from LLVM Download or building it yourself.

In this section, Clang will be used to compile LLVM libc, and finally, build and test the libc.

Create a empty build directory in C:\src or your preferred directory and cd to it using:
```
mkdir libc-build
cd libc-build
```
Run the following CMake command to generate build files. LLVM libc must be built by Clang, so ensure Clang is specified as the C and C++ compiler.
```
cmake -G Ninja ../llvm-project/llvm -DCMAKE_C_COMPILER=C:/src/clang-build/bin/clang-cl.exe -DCMAKE_CXX_COMPILER=C:/src/clang-build/bin/clang-cl.exe  -DLLVM_TARGETS_TO_BUILD=X86 -DLLVM_FORCE_BUILD_RUNTIME=libc -DLLVM_ENABLE_PROJECTS=libc -DLLVM_NATIVE_ARCH=x86_64 -DLLVM_HOST_TRIPLE=x86_64-window-x86-gnu
```
Some LLVM libc math unittests test correctness/accuracy against results from the GNU MPFR library. If you want to run math tests which use MPFR, and if MPFR on your machine is not installed in the default include and linker lookup directories, then you can specify the MPFR install directory by passing an additional CMake option as follows:

-DLLVM_LIBC_MPFR_INSTALL_PATH=<path/mpfr/install/dir>

If the above option is specified, then ${LLVM_LIBC_MPFR_INSTALL_PATH}/include will be added to the include directories, and ${LLVM_LIBC_MPFR_INSTALL_PATH}/lib will be added to the linker lookup directories.

NOTE: The GNU MPFR library depends on the GNU GMP library. If you specify the above option, then it will be assumed that GMP is also installed in the same directory or availabe in the default paths.
Build LLVM libc using:
```
ninja llvmlibc
```
Run tests using:
```
ninja checklibc
```