Did a full source build of all components (e.g. Gtk-2.4.2, pango, etc). Mostly 
works ok but eventually fails with :

/export/home/tmp2/gimp-2.0.1/app/paint-funcs/paint-funcs.c:4484: er
ing types for 'initial_region'
/export/home/tmp2/gimp-2.0.1/app/paint-funcs/paint-funcs.h:359: err
declaration of 'initial_region' was here
/export/home/tmp2/gimp-2.0.1/app/paint-funcs/paint-funcs.c:4484: er
ing types for 'initial_region'
/export/home/tmp2/gimp-2.0.1/app/paint-funcs/paint-funcs.h:359: err
declaration of 'initial_region' was here
make[3]: *** [paint-funcs.o] Error 1
make[3]: Leaving directory `/export/home/tmp2/gimp-2.0.1.bld/app/pa
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/export/home/tmp2/gimp-2.0.1.bld/app'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/export/home/tmp2/gimp-2.0.1.bld'
make: *** [all] Error 2
Make of  gimp-2.0.1  failed


Looking at the location that it is complaining about, the parameter types do 
indeed not match exactly (prototype takes Combination_Mode as last parameter 
actual code takes InitialMode as last parameter).

Guessing that older GCC versions did not detect this particlar case because 
the two types are basically the same