Fix Matchbox build

Several issues were encountered when trying to build Matchbox. This
patches fixes these issues :

 * Selecting BR2_PACKAGE_MATCHBOX should select BR2_PACKAGE_XORG7,
   otherwise the x11r7 mk files are not included, and matchbox.mk
   refers to non-existing dependencies (such as xlib_libXtst).

 * In matchbox.mk, wrong dependencies for Matchbox keyboard. When
   pango is available, use it through --enable-pango, otherwise, add
   the dependency to xlib_libXft

 * Enable AUTORECONF for libXfont and libXft, otherwise the libtool
   Buildroot patch has no effect, and these packages fail to build
   without the libtool hack.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>