cmake _MK_HAVE_FLOAT_H examples

JulNadeauCA/libagar CMakeChecks.cmake :1258

check_c_source_compiles("
#include <float.h>

int main(int argc, char *argv[]) {
	float flt = 0.0f;
	double dbl = 0.0;

	flt += FLT_EPSILON;
	dbl += DBL_EPSILON;
	return (0);
}
" _MK_HAVE_FLOAT_H)