Use the C linker, not C++ linker

Such that we don't link to libstdc++.
diff --git a/src/Makefile.am b/src/Makefile.am
index 23f5ed6..83bee49 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -49,6 +49,8 @@
 	$(NULL)
 endif
 
+CXXLINK = $(LINK)
+
 libharfbuzz_la_SOURCES = $(HBSOURCES) $(HBHEADERS)
 libharfbuzz_la_CPPFLAGS = $(GLIB_CFLAGS)
 libharfbuzz_la_LIBADD = $(GLIB_LIBS)