gcc-toolfile: do not force unused-variable as error

C++ Pre-processor ignores #pragma GCC diagnostic, thus we cannot
disable it around CLHEP headers.

There is a number of GCC bugs reported regarding this not working
and officially aggreed that it's broken, e.g.:

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53431

Signed-off-by: David Abdurachmanov <davidlt@cern.ch>