Build the HarfBuzz shaper only with ENABLE_HARFBUZZ.

harfbuzz.cpp is always in the shapers target and shaper_shim.cpp always
includes harfbuzz.hpp, which includes <hb.h>. The HarfBuzz include path is
only added with ENABLE_HARFBUZZ, so -DENABLE_HARFBUZZ=OFF fails with
'hb.h: No such file or directory' (unless the headers happen to sit on the
default search path).

Bug: https://github.com/obentoo/bentoo/issues/68

--- a/fontforgeexe/shapers/CMakeLists.txt
+++ b/fontforgeexe/shapers/CMakeLists.txt
@@ -3,8 +3,6 @@
 add_library(shapers OBJECT
   builtin.hpp
   builtin.cpp
-  harfbuzz.hpp
-  harfbuzz.cpp
   shaper_shim.hpp
   shaper_shim.cpp
 )
@@ -16,6 +14,10 @@
 )
 
 if(ENABLE_HARFBUZZ)
+  target_sources(shapers PRIVATE
+    harfbuzz.hpp
+    harfbuzz.cpp
+  )
   target_link_libraries(shapers PRIVATE
     PkgConfig::HarfBuzz
   )
--- a/fontforgeexe/shapers/shaper_shim.cpp
+++ b/fontforgeexe/shapers/shaper_shim.cpp
@@ -28,7 +28,9 @@
 
 #include "intl.h"
 #include "builtin.hpp"
+#ifdef ENABLE_HARFBUZZ
 #include "harfbuzz.hpp"
+#endif
 
 using ff::Tag;