Make 'configure' work out-of-the-box on Solaris.

Make 'configure' work out-of-the-box on Solaris.

* configure.ac: Work even if libgif etc. are missing.  Emacs works
without these libraries even if its default GUI is harmed, so they
are not strictly necessary.  Another possibility would be to
disable X11 if these libraries are missing.  The point is that a
plain 'configure' should not fail on platforms like Solaris 11
where one or more image libraries are not installed by default.