Possibly speedup cppcheck unsetting __GNUG__

src/maketools/codecheck

@@ -35,7 +35,7 @@ if [ $do_cppcheck == true ] ; then
     files="$(echo */*.{h,cpp})"
   fi
   cppcheck --std=c++11 --std=posix -j 4 --platform=unix64 --language=c++ \
-  -U__PRETTY_FUNCTION__ -U__PLUMED_HAS_EXTERNAL_LAPACK -U__PLUMED_HAS_EXTERNAL_BLAS \
+  -U__GNUG__ -U__PLUMED_HAS_EXTERNAL_LAPACK -U__PLUMED_HAS_EXTERNAL_BLAS \
   -UGMX_CYGWIN  -UF77_NO_UNDERSCORE -U_GLIBCXX_DEBUG -DNDEBUG -U__PLUMED_PBC_WHILE  \
   --template='[{file}:{line}] ({severity}) :{id}: {message}' --enable=all --inline-suppr --force \
   $files