Check compiler is gcc before pragma GCC (#157)

Author: tt4g

include/pqxx/internal/ignore-deprecated-post.hxx

@@ -1,2 +1,6 @@
 /// End a code block started by "ignore-deprecated-pre.hxx".
+#if defined(__GNUC__)
+
 #pragma GCC diagnostic pop
+
+#endif // __GNUC__

include/pqxx/internal/ignore-deprecated-pre.hxx

@@ -11,5 +11,9 @@
  * "ignore-deprecated-post.hxx".  To avoid mistakes, keep the enclosed area as
  * small as possible.
  */
+#if defined(__GNUC__)
+
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+
+#endif // __GNUC__