cross-toolset: fix galleries build with webp

All the tested cross builds don't build galleries, so the
cross-toolset won't contain gengal and no vcl is build.
To fix the full static build, add LIBWEBP to the list of
PERMITTED_BUILD_TARGETS and use gb_CONFIGURE_PLATFORMS to
provide the --host and --build flags.

Regression from commit 60eaa424c5
("support for the WebP image format (tdf#114532)").

Change-Id: I017c4fc72456859616d535ddfb2d568442ffa446
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/129790
Tested-by: Jenkins
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>

configure.ac

@@ -5667,6 +5667,7 @@ if test "$cross_compiling" = "yes"; then
         LibO
         LIBFFI
         LIBPN
+        LIBWEBP
         LIBXML2
         LIBXSLT
         MDDS

external/libwebp/ExternalProject_libwebp.mk

@@ -48,7 +48,7 @@ $(call gb_ExternalProject_get_state_target,libwebp,build) :
 			$(if $(verbose),--disable-silent-rules,--enable-silent-rules) \
 			CXXFLAGS="$(gb_CXXFLAGS) $(if $(ENABLE_OPTIMIZED),$(gb_COMPILEROPTFLAGS),$(gb_COMPILERNOOPTFLAGS))" \
 			CPPFLAGS="$(CPPFLAGS) $(BOOST_CPPFLAGS)" \
-			$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
+			$(gb_CONFIGURE_PLATFORMS) \
 		&& $(MAKE) \
 	)
 	$(call gb_Trace_EndRange,libwebp,EXTERNAL)