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

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

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. 

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