Factor configure-time gcc version checks (patch 1/4 for PR 7305)

[Andrew Haley <aph@redhat.com>]

Richard Sandiford writes:
 > So... the four patches are as follows:
 > 
 >   (1) Provide a config/gcc-version.m4 fragment to work out
 >       $gcc_version_trigger, $gcc_version_full and $gcc_version.
 > 
 >   (2) Remove config.if.
 > 
 >   (3) Provide a config/gxx-include-dir.m4 fragment and use it
 >       in the top-level configuration file.
 > 
 >   (4) The main libjava patch.
 > 
 > Part (1) is attached below.  The logic used in the new autoconf fragment
 > is mostly taken from config.if.
 > 
 > Patch bootstrapped & regression tested (on its own) on i686-pc-linux-gnu.
 > I also checked that:
 > 
 >   - The top-level configure gets the right version and version trigger.
 > 
 >   - The top-level makefile still passes --with-gcc-version-trigger
 >     down to subconfigures.
 > 
 >   - The subconfigures correctly read --with-gcc-version-trigger
 >     and record the right version number.
 > 
 >   - There's no change in the version numbers for installed paths.
 > 
 > OK to install?

Fine by me.

Andrew.