MDEV-9308 Fix build errors with recent gcc (isfinite)

"#include <math.h>" has "#define isfinite(X) ..."
while "#include <cmath>" does "#undef isfinite"

in -std=c++11 mode <cmath> is included, we need a workaround
to provide a usable isfinite()

include/my_global.h

@@ -813,6 +813,9 @@ inline unsigned long long my_double2ulonglong(double d)
 #else
 #define finite(x) (1.0 / fabs(x) > 0.0)
 #endif /* HAVE_FINITE */
+#elif (__cplusplus >= 201103L)
+#include <cmath>
+static inline bool isfinite(double x) { return std::isfinite(x); }
 #endif /* isfinite */
 
 #ifndef HAVE_ISNAN