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.c b/src/compositor.c
index 5c19d92..3b16220 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -58,6 +58,7 @@
 #include "compositor.h"
 #include "../shared/os-compatibility.h"
 #include "git-version.h"
+#include "version.h"
 
 static struct wl_list child_process_list;
 static struct weston_compositor *segv_compositor;
@@ -3046,6 +3047,14 @@
 	wl_event_loop_destroy(ec->input_loop);
 }
 
+WL_EXPORT void
+weston_version(int *major, int *minor, int *micro)
+{
+	*major = WESTON_VERSION_MAJOR;
+	*minor = WESTON_VERSION_MINOR;
+	*micro = WESTON_VERSION_MICRO;
+}
+
 static int on_term_signal(int signal_number, void *data)
 {
 	struct wl_display *display = data;