Add a check in the new configure script to enable lots of gcc warning

Add a check in the new configure script to enable lots of gcc warning
flags if gcc is in use - but _only_ if they don't lead to problems in
a test compile which includes all the system and GTK headers I'm going
to want. Ubuntu 13.04 includes a version of Pango which breaks at
-Werror -pedantic, so I can't unconditionally enable all the warning
flags I might want without breaking the build in ways beyond my
control.

[originally from svn r9888]