AP_Common: fix build on windows

The way the build was fixed for gcc >= 5.3 breaks the build for older
compiler versions on waf. Fix this by ifdef'ing for WAF_BUILD and
checking the compiler version.

libraries/AP_Common/missing/cmath

@@ -2,8 +2,10 @@
 
 #include_next <cmath>
 
-#ifdef __CYGWIN__
+#ifndef WAF_BUILD
-// hack to get SITL on windows working
+#  ifdef __GNUG__
+#    define _GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
+#    if _GCC_VERSION >= 50300
 #      ifndef HAVE_CMATH_ISFINITE
 #        define HAVE_CMATH_ISFINITE
 #        define NEED_CMATH_ISFINITE_STD_NAMESPACE
@@ -16,7 +18,10 @@
 #        define HAVE_CMATH_ISNAN
 #        define NEED_CMATH_ISNAN_STD_NAMESPACE
 #      endif
-#endif // __CYGWIN__
+#    endif
+#    undef _GCC_VERSION
+#  endif
+#endif
 
 #if defined(HAVE_CMATH_ISFINITE) && defined(NEED_CMATH_ISFINITE_STD_NAMESPACE)
 using std::isfinite;