suggestion: __has_include() wrapper, HEDLEY_HAS_INCLUDE#66

Hi, First of all: much thanks for the great library, it helped me a lot and I found it really useful. I would like to suggest a wrapper for `__has_include()` compiler built-in, a macro to let you detect, if a specific header is available. This appears to be part of C++17 and C23 standards, and is something that many compiler vendors supported for quite some time (including TCC since 2020). It is fairly easy to detect (e.g. `#if defined(__has_include)` ). The fallback could be to always evaluate to `0` in case built-in is missing (not a perfect solution but lack of __has_include was always respected as many sources have suggested to feature-test it). There is also `__has_include_next()`, but this one is not supported by any Standard, so it's out of a question. Naive implementation: ```c #if defined(__has_include) # define HEDLEY_HAS_INCLUDE(include) __has_include(include) #else # define HEDLEY_HAS_INCLUDE(include) (0) #endif ``` References: - https://en.cppreference.com/w/c/preprocessor/include - https://en.cppreference.com/w/cpp/preprocessor/include - https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005finclude.html - https://clang.llvm.org/docs/LanguageExtensions.html#langext-has-include - https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations#testing-for-the-presence-of-a-header-__has_include - https://stackoverflow.com/questions/142877/can-the-c-preprocessor-be-used-to-tell-if-a-file-exists