x11r7: do not globally select libraries

It doesn't make sense for BR2_PACKAGE_XORG7 to globally select
libraries such as libpng, zlib, expat or fontconfig. For example, if
you do a build with just xlib_libX11, then libpng gets built as the
last package, without anybody actually depending on it, even if it was
selected by BR2_PACKAGE_XORG7.

The individual x11r7 packages should select the libraries they need,
and add them in their DEPENDENCIES variable.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>