Revert changes to cmath according to gcc version

Revert these commits:
	"AP_Common: gcc 4.9.2 behaves differently from 4.9.3"
	"AP_Common: this workaround for missing/cmath is also needed for gcc 4.9.3"

This breaks the build for PX4. The change is related to the libc, not
the compiler version. We have an workaround there based on compiler
version just because it's usual to have a more recent libc version when
you have a recent compiler.

libraries/AP_Common/missing/cmath

@@ -5,7 +5,7 @@
 #ifndef WAF_BUILD
 #  ifdef __GNUG__
 #    define _GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
-#    if _GCC_VERSION >= 40903
+#    if _GCC_VERSION >= 50300
 #      ifndef HAVE_CMATH_ISFINITE
 #        define HAVE_CMATH_ISFINITE
 #        define NEED_CMATH_ISFINITE_STD_NAMESPACE