Fix build of host tool

genmtab is a tool that needs to be built for the host as it is used
during the compilation process of libxmlrpc. Its Makefile needs a bit
of tuning to use the conventional CC_FOR_BUILD, CFLAGS_FOR_BUILD and
LDFLAGS_FOR_BUILD variables.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/lib/expat/gennmtab/Makefile
===================================================================
--- a/lib/expat/gennmtab/Makefile
+++ b/lib/expat/gennmtab/Makefile
@@ -40,9 +40,9 @@
 dep: dep-common
 
 gennmtab.o:%.o:%.c
-	$(BUILDTOOL_CC) -c $< -o $@ $(CFLAGS_ALL) $(INCLUDES)
+	$(CC_FOR_BUILD) -c $< -o $@ $(CFLAGS_FOR_BUILD) $(INCLUDES)
 
 gennmtab:%:%.o
-	$(BUILDTOOL_CCLD) -o $@ $(LDFLAGS_ALL) $^
+	$(CC_FOR_BUILD) -o $@ $(LDFLAGS_FOR_BUILD) $^
 
 include depend.mk