now have ENABLE_GIMP_1_2 when GIMP 1.2 is explicitely wanted.
* configure.in: make GIMP 2.0 the default.
* include/sane/config.h: replace ENABLE_GIMP_1_3 by ENABLE_GIMP_1_2.
* aclocal.m4, acinclude.m4: include the AM_PATH_GIMP_2_0 macros,
which replaces the AM_PATH_GIMP_1_4 macro.