external/harfbuzz: Silence clang-cl -Werror,-Wmicrosoft-enum-value

The code in harfbuzz' src/hb_common.h apparently goes to some length to ensure
that any value of type hb_tag_t (aka unit32_t) can be transported as a value of
the hb_script_t enum type.  However, under MSVC any C (or non-fixed C++) enum
type has an underlying type of int, so _HB_SCRIPT_MAX_VALUE of value 0xFFFFFFFF
will cause a -Wmicrosoft-enum-value under clang-cl.  To not complicate things
further, acknowledge that converting between hb_tag_t (an unsigned integer type
with 32 value bits) and hb_script_t (a two's-complement signed integer type with
32 value bits) is well-defined under MSVC and drop _HB_SCRIPT_MAX_VALUE (which
appears to be an otherwise unused implementation detail) there.

Change-Id: Ic03dff64a9dd24683c45347fa78699708c269972

external/harfbuzz/UnpackedTarball_harfbuzz.mk

@@ -15,6 +15,7 @@ $(eval $(call gb_UnpackedTarball_set_patchlevel,harfbuzz,0))
 
 $(eval $(call gb_UnpackedTarball_add_patches,harfbuzz, \
     external/harfbuzz/ubsan.patch \
+    external/harfbuzz/clang-cl.patch \
 ))
 
 ifneq ($(ENABLE_RUNTIME_OPTIMIZATIONS),TRUE)

external/harfbuzz/clang-cl.patch

@@ -0,0 +1,12 @@
+--- src/hb-common.h
++++ src/hb-common.h
+@@ -331,7 +331,9 @@
+    *
+    *   http://lists.freedesktop.org/archives/harfbuzz/2014-March/004150.html
+    */
++#if !defined _MSC_VER /* avoid clang-cl -Wmicrosoft-enum-value */
+   _HB_SCRIPT_MAX_VALUE				= HB_TAG_MAX, /*< skip >*/
++#endif
+   _HB_SCRIPT_MAX_VALUE_SIGNED			= HB_TAG_MAX_SIGNED /*< skip >*/
+ 
+ } hb_script_t;