Wrong detection of intsafe.h at MinGW (with GCC 4.8.1)

[tbeu]

It was reported in modelica/ModelicaStandardLibrary#3556 that the following lines

portable-snippets/safe-math/safe-math.h

Lines 22 to 28 in f596f8b

	# if defined(__has_include)
	# if __has_include(<intsafe.h>)
	# define PSNIP_SAFE_HAVE_INTSAFE_H
	# endif
	# elif defined(_WIN32)
	# define PSNIP_SAFE_HAVE_INTSAFE_H
	# endif

are not protable enough for MinGW with GCC 4.8.1 (not exclusively to that version). The issue probably is, that old GCC 4.8.1 has no __has_include why it then sets PSNIP_SAFE_HAVE_INTSAFE_H in the else branch since MinGW defines _WIN32. AFAIK, there is no intsafe.h on MinGW yet.