Are you trying to install GTK2 on a Mac from MacPorts, such as for Dia?
The build dependencies are broken as of December 2008. You need to install xorg-libXrandr first.
sudo /opt/local/bin/port install xorg-libXrandr
The error you'll be seeing will be along these lines:
gdkevents-x11.c: In function 'gdk_event_translate':
gdkevents-x11.c:2110: error: 'RRNotify' undeclared (first use in this function)
gdkevents-x11.c:2110: error: (Each undeclared identifier is reported only once
gdkevents-x11.c:2110: error: for each function it appears in.)
gdkevents-x11.c:2112: error: 'XRRNotifyEvent' undeclared (first use in this function)
gdkevents-x11.c:2112: error: 'notify' undeclared (first use in this function)
gdkevents-x11.c:2112: error: syntax error before ')' token
gdkevents-x11.c: In function 'gdk_x11_screen_get_window_manager_name':
gdkevents-x11.c:2721: warning: dereferencing type-punned pointer will break strict-aliasing rules
gdkevents-x11.c: In function 'gdk_x11_screen_supports_net_wm_hint':
gdkevents-x11.c:2821: warning: dereferencing type-punned pointer will break strict-aliasing rules
make[4]: *** [gdkevents-x11.lo] Error 1
make[3]: *** [all-recursive] Error 1
make[2]: *** [all] Error 2
make[1]: *** [all-recursive] Error 1
make: *** [all] Error 2
Error: The following dependencies failed to build: gtk2 libart_lgpl libxslt popt
Error: Status 1 encountered during processing.
