Add draft experimental Uniscribe backend

Not complete yet, font selection doesn't work.  But hey it shapes!

This is not supposed to be a production backend, more like a testing
backend.
diff --git a/src/Makefile.am b/src/Makefile.am
index b812419..c4f35e9 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -108,6 +108,17 @@
 	$(NULL)
 endif
 
+if HAVE_UNISCRIBE
+HBCFLAGS += $(UNISCRIBE_CFLAGS)
+HBLIBS   += $(UNISCRIBE_LIBS)
+HBSOURCES += \
+	hb-uniscribe-shape.cc \
+	$(NULL)
+HBHEADERS += \
+	hb-uniscribe.h \
+	$(NULL)
+endif
+
 CXXLINK = $(LINK)
 libharfbuzz_la_SOURCES = $(HBSOURCES) $(HBHEADERS)
 libharfbuzz_la_CPPFLAGS = $(HBCFLAGS)