[llvm] [flang-rt] Added FLANG_RT_QUADMATH_INCLUDE_DIR option. (PR #130378)

[Slava Zakharin via llvm-commits]

vzakhari wrote:

> Why not just `-DCMAKE_CXX_FLAGS=-I/usr//lib/gcc/x86_64-linux-gnu/13/include`? No additional configuration complexity needed.
> 
> Clang is already searching for gcc installations, just doesn't add its include dir to the default header search path.

`check_include_file` does not rely on `CMAKE_C/CXX_FLAGS` for compiling the sample code.

https://github.com/llvm/llvm-project/pull/130378