Install header files and pkg-config file for external modules

This patch installs the three header files that define the compositor
plugin interface as well as a pkg-config file.  This allows
building weston plugins outside the weston tree.  We currently don't make
any guarantees about the plugin API/ABI except that within a stable
branch we won't break it.

src/xwayland/Makefile.am

@@ -1,4 +1,5 @@
 AM_CPPFLAGS =					\
+	-I$(top_srcdir)/shared			\
 	-DDATADIR='"$(datadir)"'		\
 	-DMODULEDIR='"$(moduledir)"'		\
 	-DLIBEXECDIR='"$(libexecdir)"'		\