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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

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

>From the [documention](https://cmake.org/cmake/help/latest/module/CheckIncludeFile.html):
> `CMAKE_REQUIRED_FLAGS`
> String of additional flags to pass to the compiler. The string must be space-delimited--a [;-list](https://cmake.org/cmake/help/latest/manual/cmake-language.7.html#cmake-language-lists) will not work. The contents of [`CMAKE_<LANG>_FLAGS`](https://cmake.org/cmake/help/latest/variable/CMAKE_LANG_FLAGS.html#variable:CMAKE_%3CLANG%3E_FLAGS) and its associated configuration-specific variable are automatically added to the compiler command before the contents of CMAKE_REQUIRED_FLAGS.


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