This bug is about automatic dependences used in gnome-control-center's configure.ac file.

Please use either mandatory dependencies and optional dependencies only.
The prefered way for the source based distributions is to deal with optional dependencies.

The automatic dependencies on the other side are evil! A more detailed explanation can be found here: http://www.gentoo.org/proj/en/qa/automagic.xml

Here is an example of automatic dependence from the configure.ac file:

# Check for libmm-glib for ModemManager1 support
PKG_CHECK_MODULES(MM_GLIB, mm-glib,
                  [have_libmm_glib=yes], have_libmm_glib=no)
if test "x$have_libmm_glib" = xno ; then
  AC_MSG_WARN(*** Network panel will not be built with ModemManager1 support (libmm-glib not found) ***)
else
  AC_DEFINE(HAVE_MM_GLIB, 1, [Defined if libmm-glib is available])
fi
AM_CONDITIONAL(HAVE_MM_GLIB, test "x$have_libmm_glib" = "xyes")

For now the issues regarding the stabilization of the modemmanager (https://bugzilla.gnome.org/show_bug.cgi?id=688238#c24 also on binary-based distributions https://bugs.launchpad.net/ubuntu/+source/modemmanager/+bug/1123302), combined with the automatic dependencies issues turned into in one of the BLOCKERS, which makes the integration of GNOME 3.8.x especially on gentoo very hard. Therefore, this is one of the reasons, why GNOME 3.8 is hard masked on gentoo.

Based on the above said, please do not make the dependency on modemmanager mandatory!!!