diff -up gdm-2.29.6/configure.ac.libs gdm-2.29.6/configure.ac
--- gdm-2.29.6/configure.ac.libs	2010-02-11 19:25:51.485272530 -0500
+++ gdm-2.29.6/configure.ac	2010-02-11 19:27:05.805022498 -0500
@@ -145,6 +145,9 @@ PKG_CHECK_MODULES(SIMPLE_GREETER,
 	fontconfig >= $FONTCONFIG_REQUIRED_VERSION
 	x11
 )
+
+SIMPLE_GREETER_LIBS="$SIMPLE_GREETER_LIBS -lm"
+
 AC_SUBST(SIMPLE_GREETER_CFLAGS)
 AC_SUBST(SIMPLE_GREETER_LIBS)