Get rid of reproducible build warning by removing build date & time from flatc version (#5086)

* disable reproducible build warning due to date/time macros

* wrapped GCC pragmas in #ifdef _GNUC_

* removed __DATE__ and __TIME__ macros from flatc.cpp

src/flatc.cpp

@@ -18,7 +18,7 @@
 
 #include <list>
 
-#define FLATC_VERSION "1.10.0 (" __DATE__ " " __TIME__ ")"
+#define FLATC_VERSION "1.10.0"
 
 namespace flatbuffers {