libgomp: Use absolute pathname to testsuite/flock [PR113192]

When flock program doesn't exist, libgomp configure attempts to
offer a fallback version using a perl script, but we weren't using
absolute filename to that, so it apparently failed to work correctly.

The following patch arranges for it to get the absolute filename.

Tested by John David in the PR.

2024-01-09  Jakub Jelinek  <jakub@redhat.com>

	PR libgomp/113192
	* configure.ac (FLOCK): Use \$(abs_top_srcdir)/testsuite/flock
	rather than $srcdir/testsuite/flock.
	* configure: Regenerated.