Pass POSIX_C_SOURCE for std::alligned_alloc try_compile checks

Pass POSIX_C_SOURCE for std::alligned_alloc try_compile checks

On FreeBSD 11, definining POSIX_C_SOURCE to a lower level has the efect of inhibiting the language level (__ISO_C_VISIBLE ) to be lower than C11, even in the presence of -std=c11

Since the check_symbol_exists runs without setting POSIX_C_SOURCE, this means that we will spuriously define HAVE_ALIGNED_ALLOC, while in the actual build it is not going to be defined

ref: https://github.com/freebsd/freebsd-src/blob/stable/11/sys/sys/cdefs.h#L738