disable ccache for PCH generation

Ccache can't handle it well with -gsplit-dwarf, and it's a question
if it even makes sense to spend cache space on large PCHs.
https://github.com/ccache/ccache/issues/419

Change-Id: I9a458c1793fcd503f7d29ea76c6e7651ab378783
Reviewed-on: https://gerrit.libreoffice.org/72041Reviewed-by: Luboš Luňák <l.lunak@collabora.com>
Tested-by: Luboš Luňák <l.lunak@collabora.com>

solenv/gbuild/platform/com_GCC_class.mk

@@ -94,6 +94,7 @@ define gb_PrecompiledHeader__command
 $(call gb_Output_announce,$(2),$(true),PCH,1)
 $(call gb_Helper_abbreviate_dirs,\
 	mkdir -p $(dir $(1)) $(dir $(call gb_PrecompiledHeader_get_dep_target,$(2),$(7))) && \
+	CCACHE_DISABLE=1 $(gb_COMPILER_SETUP) \
 	$(gb_CXX) \
 		-x c++-header \
 		$(4) $(5) \