gtk+: Build from source.

This bunch of changes is required to build gtk+ from source and to
update to the latest gtk+-2 version.

• m4/gpg4win.m4: Support tar.xz files.
• src/zlib.pc: New.
• src/gpg4win.mk.in: Support tar.xz. Support pkgextracflags macro.

• configure.ac: Gurn gtk+ into a source package. Add libffi and

gdk-pixbuf packages. Print warning for missing tools required by glib
et al.

• include/config.nsi.in: Add packages libffi and gdk-pixbuf.
• src/inst-libffi.nsi, src/uninst-libffi.nsi: New.
• src/inst-gdk-pixbuf.nsi, src/uninst-gdk-pixbuf.nsi: New.

• src/inst-cairo.nsi (libcairo-gobject-2.dll)

(libcairo-script-interpreter-2.dll): Install new DLLs.

• src/uninst-cairo.nsi: Uninstall them.

• src/inst-glib.nsi: Install more tools and the schema.
• src/uninst-glib.nsi: Uninstall them.