Portcheck only if --enable-gcc-warnings

Problem reported by Richard Copley (Bug#37852).
This patch causes the problem to not occur if one uses plain
‘configure’.  The problem can still occur if with ‘configure
--enable-gcc-warnings’, so a further fix may be needed.
* configure.ac (GNULIB_PORTCHECK, _FORTIFY_SOURCE):
Define these only with an explicit --enable-gcc-warnings.
1 job for master in 57 minutes and 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #3934
test-all

00:57:09