libmatchbox: fix libpng detection

The configure script looks for libpng12 though pkg-config and if that fails
falls back to looking for library files directly.  The result of this is that
the linkage can change between libpng12 or libpng16 depending on what is
installed in the sysroot.

To resolve this, take a patch from upstream to just link using pkg-config.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
libmatchbox: use PACKAGECONFIG

Use PACKAGECONFIG to offer some flexibility to the libmatchbox configuration,
and remove two spurious build dependencies (expat and libstartup-notification).

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>