Introduce --enable-headless to add it as a possibility on macOS

Change-Id: I3ea0fbdab2d36bf9c9565ed3274eee978ba6764d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/194791
Reviewed-by: Caolán McNamara <caolan.mcnamara@collabora.com>
Tested-by: Caolán McNamara <caolan.mcnamara@collabora.com>

RepositoryExternal.mk

@@ -1280,7 +1280,7 @@ endef
 
 else # !SYSTEM_CAIRO
 
-ifneq ($(filter-out MACOSX WNT,$(OS)),)
+ifneq ($(filter-out MACOSX$(ENABLE_HEADLESS) WNT,$(OS)),)
 
 $(eval $(call gb_Helper_register_packages_for_install,ooo,\
 	cairo \

configure.ac

@@ -1077,7 +1077,11 @@ cygwin*|wsl*)
     ;;
 
 darwin*) # macOS
-    using_freetype_fontconfig=no
+    if test "$enable_headless" != "yes"; then
+        using_freetype_fontconfig=no
+    else
+        using_freetype_fontconfig=yes
+    fi
     using_x11=no
     build_skia=yes
     enable_pagein=no
@@ -1934,6 +1938,11 @@ AC_ARG_ENABLE(gen,
          Per default auto-enabled when X11 is used.]),
 ,test "${test_gen}" = no -o "${enable_gen+set}" = set || enable_gen=yes)
 
+AC_ARG_ENABLE(headless,
+    AS_HELP_STRING([--enable-headless],
+        [Enable headless plugin (svp) even on macOS.]),
+,using_headless_plugin=yes)
+
 AC_ARG_ENABLE(gui,
     AS_HELP_STRING([--disable-gui],
         [Disable use of X11 or Wayland to reduce dependencies (e.g. for building LibreOfficeKit).]),