I found a neat Clang-specific feature that lets you know if a header exists before actually including it (__has_include). I was trying to come up with my own macro to do something like the following:
#define DOES_HEADER_EXIST(_header_) \
#if !__has_include(_header_) \
#warning Header not found: _header_ \
#endif
but this does not seem to work. I can see why this would not work, as it's kind of like asking the preprocessor to do two passes, but maybe I'm wrong?
There is no way to create a macro that will expand into a preprocessor directive that the preprocessor will actually take action on.
Your particular macro is flawed because it expands into a single long line that would have been considered a malformed #if directive, if the compiler would have allowed the # to be followed by a token that wasn't a macro parameter.
To do what you want, you will need a pre-preprocessor that injects the tests you want to perform before you actually compile the code.