Fix dependency management of pch files

We were using the same .d name for the debug and release
versions of the pch, which would cause them to not get rebuilt
sometimes, and then clang would complain.

Makefile

@@ -647,7 +647,7 @@ $$(SRCS:.cpp=$1.o.bc): CXXFLAGS:=$2
 %$1.h.pch: %.h $$(BUILD_SYSTEM_DEPS)
 	$$(ECHO) Compiling $$@
 	$$(VERB) rm -f $$@-*
-	$$(VERB) $$(CLANGPP_EXE) $$(CXXFLAGS) -MMD -MP -MF $$<.d -x c++-header $$< -o $$@
+	$$(VERB) $$(CLANGPP_EXE) $$(CXXFLAGS) -MMD -MP -MF $$(patsubst %.pch,%.d,$$@) -x c++-header $$< -o $$@
 $$(CODEGEN_SRCS:.cpp=$1.o): CXXFLAGS += -include src/codegen/irgen$1.h
 $$(CODEGEN_SRCS:.cpp=$1.o): src/codegen/irgen$1.h.pch