Fix combreloc test with BSD grep

The test for "-z combreloc" fails when cross-compiling on a machine
that uses BSD grep (e.g. on macos). grep complains about empty
subexpression and exits with non-zero status, which is interpreted
by configure as "not found". As a result, support for "-z combreloc"
(HAVE_Z_COMBRELOC) is not detected, leading to link failure on SPARC.

While there, replace fgrep with 'grep -F', as fgrep is non-POSIX.

	* configure.ac: Avoid empty subexpression in grep.

Signed-off-by: Alexey Neyman <stilor@att.net>