Allow building against system-wide graphite2/harfbuzz.
See <https://bugzilla.mozilla.org/show_bug.cgi?id=847568>
Based on:
  https://svnweb.freebsd.org/ports/head/www/firefox-esr/files/patch-bug847568?revision=472833&view=co
Modified for use with patch -p1, and to apply cleanly to GNU IceCat.

--- icecat-60.5.0/config/system-headers.mozbuild
+++ icecat-60.5.0/config/system-headers.mozbuild
@@ -1311,6 +1311,19 @@
         'pixman.h',
     ]
 
+if CONFIG['MOZ_SYSTEM_GRAPHITE2']:
+    system_headers += [
+        'graphite2/Font.h',
+        'graphite2/Segment.h',
+    ]
+
+if CONFIG['MOZ_SYSTEM_HARFBUZZ']:
+    system_headers += [
+        'harfbuzz/hb-glib.h',
+        'harfbuzz/hb-ot.h',
+        'harfbuzz/hb.h',
+    ]
+
 if CONFIG['MOZ_SYSTEM_LIBVPX']:
     system_headers += [
         'vpx_mem/vpx_mem.h',
--- icecat-60.5.0/dom/base/moz.build
+++ icecat-60.5.0/dom/base/moz.build
@@ -474,6 +474,9 @@
 if CONFIG['MOZ_X11']:
     CXXFLAGS </td></tr></table>
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