Make it possible to disable the fallback shaper at configure time

The OT shaper supersedes the fallback shaper in every case
and the latter become an extra weight for 99.9% of users.
diff --git a/src/Makefile.am b/src/Makefile.am
index c650b11..e2790a0 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -27,7 +27,6 @@
 	hb-buffer.cc \
 	hb-cache-private.hh \
 	hb-common.cc \
-	hb-fallback-shape.cc \
 	hb-face-private.hh \
 	hb-face.cc \
 	hb-font-private.hh \
@@ -119,6 +118,10 @@
 	$(NULL)
 endif
 
+if HAVE_FALLBACK
+HBSOURCES += hb-fallback-shape.cc
+endif
+
 if HAVE_PTHREAD
 HBCFLAGS += $(PTHREAD_CFLAGS)
 HBLIBS   += $(PTHREAD_LIBS)