mutter-launch: make sure that the spawned binaries sees the right libraries

Being a setuid binary, our LD_LIBRARY_PATH is cleared by glibc at
startup, but we need the spawned binary to see it, otherwise
jhbuild doesn't work, so hardcode it using the configured libdir.

https://bugzilla.gnome.org/show_bug.cgi?id=705861

src/Makefile.am

@@ -251,7 +251,7 @@ bin_PROGRAMS+=mutter-launch
 
 mutter_launch_SOURCES = wayland/weston-launch.c wayland/weston-launch.h
 
-mutter_launch_CFLAGS = $(MUTTER_LAUNCH_CFLAGS)
+mutter_launch_CFLAGS = $(MUTTER_LAUNCH_CFLAGS) -DLIBDIR=\"$(libdir)\"
 mutter_launch_LDFLAGS = $(MUTTER_LAUNCH_LIBS) -lpam
 
 install-exec-hook:

src/wayland/weston-launch.c

@@ -433,7 +433,7 @@ launch_compositor(struct weston_launch *wl, int argc, char *argv[])
 		setenv_fd("WESTON_TTY_FD", wl->tty);
 
 	setenv_fd("WESTON_LAUNCHER_SOCK", wl->sock[1]);
-
+	setenv("LD_LIBRARY_PATH", LIBDIR, 1);
 	unsetenv("DISPLAY");
 
 	/* Do not give our signal mask to the new process. */