configure, jit: Allow for 'make check-gcc-jit'.

This is a convenience feature that allows the user to
do "make check-gcc-jit" at the top level of the build
to check that facility in isolation from others.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

ChangeLog:

	* Makefile.def: Add a jit check target for the jit
	language.
	* Makefile.in: Regenerate.