remove PCH file before generating it

Compilers do not necessarily output all includes files when included
using a PCH, so when a PCH is being rebuilt, it might actually get
reused this way, causing its dependencies to be incomplete afterwards.
The PCH actually should not be used when being regenerated, but with MSVC
this apparently happens, even though there's only the option to create
PCH, not to use it.

Change-Id: Iee1340a60085550ab867304c44752daa04cbba31

solenv/gbuild/PrecompiledHeaders.mk

@@ -42,9 +42,11 @@ $(call gb_NoexPrecompiledHeader_get_dep_target,%) :
 		echo "$(call gb_NoexPrecompiledHeader_get_target,$*) : $(gb_Helper_PHONY)" > $@)
 
 $(call gb_PrecompiledHeader_get_target,%) :
+	rm -f $@
 	$(call gb_PrecompiledHeader__command,$@,$*,$<,$(PCH_DEFS),$(PCH_CXXFLAGS) $(gb_PrecompiledHeader_EXCEPTIONFLAGS),$(INCLUDE))
 
 $(call gb_NoexPrecompiledHeader_get_target,%) :
+	rm -f $@
 	$(call gb_NoexPrecompiledHeader__command,$@,$*,$<,$(PCH_DEFS),$(PCH_CXXFLAGS) $(gb_NoexPrecompiledHeader_NOEXCEPTIONFLAGS),$(INCLUDE))
 
 .PHONY : $(call gb_PrecompiledHeader_get_clean_target,%) $(call gb_NoExPrecompiledHeader_get_clean_target,%)