Don't mis-apply GCC < 4.6 workaround for Clang

...which happens to define __GNUC__=4, __GNUC_MINOR__=2.  (See also
<https://git.libreoffice.org/core/+/9a23239b710e44754e4a28643420c9ba91552645%5E%21>
"external/cppunit: Don't mis-apply GCC < 4.6 workaround for Clang".  This change
requires <https://gerrit.libreoffice.org/c/cppunit/+/147383> "Use snprintf
instead of sprintf" to avoid deprecation warnings on macOS, which had originally
been hidden by the mis-applied #pragma.)

Change-Id: I4fcb38a79766238862795ba4019638862e65b041
Reviewed-on: https://gerrit.libreoffice.org/c/cppunit/+/147384


Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>
Tested-by: Markus Mohrhard <markus.mohrhard@googlemail.com>