Do not use GCC pragmas for Clang

Clang defines __GNUC__ then complains it doesn't know the
warning options from GCC since it doesn't know -Wpragmas
either:

core/clingutils/res/TClingUtils.h:28:32: warning:
 unknown warning group '-Wclass-memaccess', ignored [-Wunknown-warning-option]
 #pragma GCC diagnostic ignored "-Wclass-memaccess"