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.
diff --git a/src/compositor.h b/src/compositor.h
index c0694e8..c39abe3 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -28,8 +28,9 @@
 #include <xkbcommon/xkbcommon.h>
 #include <wayland-server.h>
 
-#include "../shared/matrix.h"
-#include "../shared/config-parser.h"
+#include "version.h"
+#include "matrix.h"
+#include "config-parser.h"
 
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
 
@@ -496,6 +497,9 @@
 };
 
 void
+weston_version(int *major, int *minor, int *micro);
+
+void
 weston_surface_update_transform(struct weston_surface *surface);
 
 void