Automatically generate weston.ini with the right paths

Previously weston.ini had hardcoded paths for the weston-* clients in
/usr/bin and /usr/libexec. This was a bit annoying when testing Weston
because you wouldn't usually install those in the system prefix. This
patch adds a make rule to automatically generate weston.ini from a
template file with some replacement markers for the paths so that they
can have the right prefix.
diff --git a/Makefile.am b/Makefile.am
index e9ecc38..3a7d337 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,4 +6,13 @@
 
 DISTCHECK_CONFIGURE_FLAGS = --disable-setuid-install
 
-EXTRA_DIST = weston.ini wayland-scanner.mk
+EXTRA_DIST = weston.ini.in wayland-scanner.mk
+
+weston.ini : $(srcdir)/weston.ini.in
+	$(AM_V_GEN)$(SED) \
+		-e 's|@bindir[@]|$(bindir)|g' \
+		-e 's|@abs_top_builddir[@]|$(abs_top_builddir)|g' \
+		-e 's|@libexecdir[@]|$(libexecdir)|g' \
+		$< > $@
+
+all-local : weston.ini