Avoid GCC diagnostic pragma if GCC < 4.6

GCC diagnostic pragmas were introduced in GCC 4.6, so augment the GCC
compiler check with a version check to avoid compilation errors with GCC
< 4.6.

Cython/Utility/TypeConversion.c

@@ -710,6 +710,13 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
 
 // NOTE: inlining because most arguments are constant, which collapses lots of code below
 
+// GCC diagnostic pragmas were introduced in GCC 4.6
+#ifdef __GNUC__
+#define GCC_VERSION (__GNUC__ * 10000 + __GNUC_MINOR__ * 100 + __GNUC_PATCHLEVEL__)
+#if GCC_VERSION >= 40600
+#define GCC_DIAGNOSTIC
+#endif
+#endif
 static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t width, char padding_char, char format_char) {
     // simple and conservative C string allocation on the stack: each byte gives at most 3 digits, plus sign
     char digits[sizeof({{TYPE}})*3+2];
@@ -719,12 +726,12 @@ static CYTHON_INLINE PyObject* {{TO_PY_FUNCTION}}({{TYPE}} value, Py_ssize_t wid
     Py_ssize_t length, ulength;
     int prepend_sign, last_one_off;
     {{TYPE}} remaining;
-#ifdef __GNUC__
+#ifdef GCC_DIAGNOSTIC
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wconversion"
 #endif
     const {{TYPE}} neg_one = ({{TYPE}}) -1, const_zero = ({{TYPE}}) 0;
-#ifdef __GNUC__
+#ifdef GCC_DIAGNOSTIC
 #pragma GCC diagnostic pop
 #endif
     const int is_unsigned = neg_one > const_zero;