AP_Common: change check from GCC version to glibc version (and Cygwin)

libraries/AP_Common/missing/cmath

@@ -3,23 +3,29 @@
 #include_next <cmath>
 
 #ifndef WAF_BUILD
-#  ifdef __GNUG__
+#  ifdef __GLIBC__
-#    define _GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
+#    if __GLIBC_PREREQ(2, 23)
-#    if _GCC_VERSION >= 50300
+#      define GLIBC_FIX_NEEDED 1
-#      ifndef HAVE_CMATH_ISFINITE
+#    else
-#        define HAVE_CMATH_ISFINITE
+#      define GLIBC_FIX_NEEDED 0
-#        define NEED_CMATH_ISFINITE_STD_NAMESPACE
-#      endif
-#      ifndef HAVE_CMATH_ISINF
-#        define HAVE_CMATH_ISINF
-#        define NEED_CMATH_ISINF_STD_NAMESPACE
-#      endif
-#      ifndef HAVE_CMATH_ISNAN
-#        define HAVE_CMATH_ISNAN
-#        define NEED_CMATH_ISNAN_STD_NAMESPACE
-#      endif
 #    endif
-#    undef _GCC_VERSION
+#  else
+#    define GLIBC_FIX_NEEDED 0
+#  endif
+#  if GLIBC_FIX_NEEDED || defined (__CYGWIN__)
+#    ifndef HAVE_CMATH_ISFINITE
+#      define HAVE_CMATH_ISFINITE
+#      define NEED_CMATH_ISFINITE_STD_NAMESPACE
+#    endif
+#    ifndef HAVE_CMATH_ISINF
+#      define HAVE_CMATH_ISINF
+#      define NEED_CMATH_ISINF_STD_NAMESPACE
+#    endif
+#    ifndef HAVE_CMATH_ISNAN
+#      define HAVE_CMATH_ISNAN
+#      define NEED_CMATH_ISNAN_STD_NAMESPACE
+#    endif
+#  undef GLIBC_FIX_NEEDED
 #  endif
 #endif