Add option to enable HarfBuzz support independent of platform

Make HarfBuzz compile on any platform using the --with-harfbuzz
option. Support is experimental only.

Change-Id: I84fb80f3f8abed8ac877a294cf7ef39cf4cb2e9e
Reviewed-on: https://gerrit.libreoffice.org/25369Reviewed-by: Khaled Hosny <khaledhosny@eglug.org>
Tested-by: Khaled Hosny <khaledhosny@eglug.org>

configure.ac

@@ -2131,6 +2131,12 @@ AC_ARG_WITH(iwyu,
          Use only if you are hacking on it.]),
 ,)
 
+AC_ARG_WITH(harfbuzz,
+    AS_HELP_STRING([--with-harfbuzz],
+        [Enable HarfBuzz support regardless of the platform.
+         Experimental only. Use only if working on it.]),
+,)
+
 dnl ===================================================================
 dnl Branding
 dnl ===================================================================
@@ -9205,7 +9211,7 @@ dnl ===================================================================
 dnl HarfBuzz
 dnl ===================================================================
 AC_MSG_CHECKING([whether to enable HarfBuzz support])
-if test $_os != WINNT -a $_os != Darwin -a $_os != iOS; then
+if test "$with_harfbuzz" = "yes" -o \( $_os != WINNT -a $_os != Darwin -a $_os != iOS \); then
     AC_MSG_RESULT([yes])
     ENABLE_HARFBUZZ="TRUE"
     if $PKG_CONFIG --atleast-version 0.9.18 harfbuzz; then

external/harfbuzz/ExternalProject_harfbuzz.mk

@@ -21,7 +21,8 @@ $(eval $(call gb_ExternalProject_use_externals,harfbuzz,\
 
 $(call gb_ExternalProject_get_state_target,harfbuzz,build) :
 	$(call gb_ExternalProject_run,build,\
-		$(if $(CROSS_COMPILING),PATH=$(SRCDIR)/external/icu/cross-bin:$$PATH) \
+		$(if $(CROSS_COMPILING),ICU_CONFIG=$(SRCDIR)/external/icu/cross-bin/icu-config) \
+		$(if $(SYSTEM_ICU),,ICU_CONFIG=$(SRCDIR)/external/icu/cross-bin/icu-config) \
 		./configure \
 			--enable-static \
 			--disable-shared \