The output is:

gcc -DHAVE_CONFIG_H -I. -I. -I../.. -I. -I.. -I../../vicious-extensions -DGNOMELOCALEDIR=\"/opt/gnome/share/locale\" -DXTHREADS -D_REENTRANT -DXUSE_MTSAFE_API -pthread -DORBIT2=1 -I/usr/include/gtk-2.0 -I/usr/lib/gtk-2.0/include -I/usr/X11R6/include -I/usr/include/atk-1.0 -I/usr/include/pango-1.0 -I/usr/include/freetype2 -I/usr/include/freetype2/config -I/usr/include/glib-2.0 -I/usr/lib/glib-2.0/include -I/usr/include/libart-2.0 -I/usr/include/libxml2 -I/usr/include/libglade-2.0 -I/opt/gnome/include/libgnomeui-2.0 -I/opt/gnome/include/libgnome-2.0 -I/opt/gnome/include/libgnomecanvas-2.0 -I/opt/gnome/include/gconf/2 -I/opt/gnome/include/libbonoboui-2.0 -I/opt/gnome/include/orbit-2.0 -I/opt/gnome/include/libbonobo-2.0 -I/opt/gnome/include/gnome-vfs-2.0 -I/opt/gnome/lib/gnome-vfs-2.0/include -I/opt/gnome/include/bonobo-activation-2.0 -march=pentium3 -Os -fomit-frame-pointer -s -pipe -DNDEBUG -DB_DISABLE_ASSERT -Wall -Wmissing-prototypes -MT keymouselistener.lo -MD -MP -MF .deps/keymouselistener.Tpo -c keymouselistener.c -fPIC -DPIC -o .libs/keymouselistener.o
keymouselistener.c: In function `event_time':
keymouselistener.c:524: error: `XDeviceKeyEvent' undeclared (first use in this function)
keymouselistener.c:524: error: (Each undeclared identifier is reported only once
keymouselistener.c:524: error: for each function it appears in.)
keymouselistener.c:524: error: parse error before ')' token
keymouselistener.c:527: error: `XDeviceButtonEvent' undeclared (first use in this function)
keymouselistener.c:527: error: parse error before ')' token
keymouselistener.c: In function `keycodes_equal':
keymouselistener.c:549: error: `XDeviceKeyEvent' undeclared (first use in this function)
keymouselistener.c:549: error: parse error before ')' token
keymouselistener.c:549: error: parse error before ')' token
keymouselistener.c: In function `key_gesture_compare_func':
keymouselistener.c:580: error: `XDeviceButtonEvent' undeclared (first use in this function)
keymouselistener.c:580: error: parse error before ')' token
keymouselistener.c: In function `gestures_filter':
keymouselistener.c:603: warning: unused variable `xinput_device'
make[3]: *** [keymouselistener.lo] Error 1
make[3]: Leaving directory `/var/abs/extra/gnome/gdm/src/gdm-2.6.0.9/gui/modules'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/var/abs/extra/gnome/gdm/src/gdm-2.6.0.9/gui'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/var/abs/extra/gnome/gdm/src/gdm-2.6.0.9'
make: *** [all] Error 2
==> ERROR: Build Failed. Aborting...
Error: Failed to build "gdm"

I try with my flags:
export CFLAGS="-march=pentium3 -Os -fomit-frame-pointer -s -pipe -DNDEBUG -DB_DISABLE_ASSERT"
export CXXFLAGS="-march=pentium3 -Os -fomit-frame-pointer -s -pipe -DNDEBUG -DB_DISABLE_ASSERT"

And with flags:
export CFLAGS="-mcpu=i686 -O2 -pipe"
export CXXFLAGS="-mcpu=i686 -O2 -pipe"


On both same result