So, Jeroen’s post had me a bit worried, having just started building GNOME on some dual opterons we received at Novell recently. So, I set up jhbuild on internal 64 bit SuSE snapshots which use gcc4. I was also a bit brash and added os.environ['MAKEFLAGS'] = '-j 4' to my .jhbuildrc. I then did a most of the way initial run to make sure I had libpng, zlib, etc devel packages all in place. Next I killed the source and install dirs and then started over. I skipped building mozilla and just used the packaged one, this did cause a small problem installing the totem plugin. Here are the rest of the issues I found (with hacky solutions in brackets):
gconf
-make -j 4 fails in gconf/backends (finish with plain make)
dbus
-make -j 4 fails in dbus/glib (finish with plain make)
Libgtop
-linking statically to Xauth, only ship libXau.a (hand fixed configure.in xauth check, disables remote MIT cookie support)
gstreamer
-ignores libdir arg from autogen (made common/gst-autogen.sh pass libdir args)
gst-plugins
-warnings as errors (hand edited configure.in to get rid of -Werror)
-ignores libdir arg from autogen (made common/gst-autogen.sh pass libdir args)
gnome-applets
-make -j 4 failed (finish with plain make)
gnome-doc-utils
-build failed creating manpage (use docbook-to-man instead of docbook2x-man)
eog
-configure check is broken for exif, i seem to get libexif >= 0.5.12 and libexif = 0.5.12 in the EOG check, not sure why because i only have pkg-config files for libexif 0.6.13 installed (made it only look for older exif if newer exif was not found)
gnome-games
-G_INLINE_FUNC evalutates to an extern declaration on my system and the linker blows up trying to find the symbol (removed G_INLINE_FUNC macro)
libmusicbrainz
-cast from ‘char*’ to ‘int’ loses precision (cast to long instead)
gnome-meeting
-me being lazy and not fiddling to have the sdl libraries found
evince
-ps-document.c bailed on locale issues (included locale.h)
All in all not too bad, YMMV. I suspect this stuff could vary more widely distro to distro than 32bit stuff.