build: Build the DirectWrite sources if requested

This adds to the autotools build system so that the (experimental)
DirectWrite support for HarfBuzz is built (and dist'ed).

src/Makefile.am

diff --git a/src/Makefile.am b/src/Makefile.am
index 9b0affa..bb085ad 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -78,6 +78,13 @@
 HBHEADERS += $(HB_UNISCRIBE_headers)
 endif
 
+if HAVE_DIRECTWRITE
+HBCFLAGS += $(DIRECTWRITE_CXXFLAGS)
+HBNONPCLIBS += $(DIRECTWRITE_LIBS)
+HBSOURCES += $(HB_DIRECTWRITE_sources)
+HBHEADERS += $(HB_DIRECTWRITE_headers)
+endif
+
 if HAVE_CORETEXT
 HBCFLAGS += $(CORETEXT_CFLAGS)
 HBNONPCLIBS += $(CORETEXT_LIBS)