Makefiles: Allow bindings to be omitted from libmctp.a

Some environments may want to only enable specific bindings.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
diff --git a/Makefile.inc b/Makefile.inc
index 99edaac..98df834 100644
--- a/Makefile.inc
+++ b/Makefile.inc
@@ -1,5 +1,8 @@
 LIBMCTP_DIR ?= libmctp/
-LIBMCTP_OBJS = core.o alloc.o serial.o
+LIBMCTP_OBJS = core.o alloc.o
+LIBMCTP_BINDINGS ?= serial
+
+LIBMCTP_OBJS += $(LIBMCTP_BINDINGS:%=%.o)
 
 LIBMCTP = $(LIBMCTP_DIR)libmctp.a