Fix compile with older FreeType

configure.ac

@@ -38,6 +38,13 @@ AM_CONDITIONAL(HAVE_ICU, $have_icu)
 PKG_CHECK_MODULES(FREETYPE, freetype2, have_freetype=true, have_freetype=false)
 if $have_freetype; then
 	AC_DEFINE(HAVE_FREETYPE, 1, [Have FreeType 2 library])
+	_save_libs="$LIBS"
+	_save_cflags="$CFLAGS"
+	LIBS="$LIBS $FREETYPE_LIBS"
+	CFLAGS="$CFLAGS $FREETYPE_CFLAGS"
+	AC_CHECK_FUNCS(FT_Face_GetCharVariantIndex)
+	LIBS="$_save_libs"
+	CFLAGS="$_save_cflags"
 fi
 AM_CONDITIONAL(HAVE_FREETYPE, $have_freetype)
 

src/hb-ft.c

@@ -39,11 +39,13 @@ hb_ft_get_glyph (hb_font_t *font, hb_face_t *face, const void *user_data,
 {
   FT_Face ft_face = (FT_Face) user_data;
 
+#ifdef HAVE_FT_FACE_GETCHARVARIANTINDEX
   if (HB_UNLIKELY (variation_selector)) {
     hb_codepoint_t glyph = FT_Face_GetCharVariantIndex (ft_face, unicode, variation_selector);
     if (glyph)
       return glyph;
   }
+#endif
 
   return FT_Get_Char_Index (ft_face, unicode);
 }