Fix precompiled-headers generation

It's now regenerated when util.hh changes, and is ordered after
config.h to fix a race.

local.mk

@@ -10,3 +10,5 @@ GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I
 
 $(foreach i, config.h $(call rwildcard, src/lib*, *.hh), \
  $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))
+
+$(GCH) $(PCH): src/libutil/util.hh config.h

mk/precompiled-headers.mk

@@ -8,14 +8,14 @@ GCH = $(buildprefix)precompiled-headers.h.gch
 $(GCH): precompiled-headers.h
  @rm -f $@
  @mkdir -p "$(dir $@)"
- $(trace-gen) $(CXX) -x c++-header -o $@ $^ $(GLOBAL_CXXFLAGS)
+ $(trace-gen) $(CXX) -x c++-header -o $@ $< $(GLOBAL_CXXFLAGS)
 
 PCH = $(buildprefix)precompiled-headers.h.pch
 
 $(PCH): precompiled-headers.h
  @rm -f $@
  @mkdir -p "$(dir $@)"
- $(trace-gen) $(CXX) -x c++-header -o $@ $^ $(GLOBAL_CXXFLAGS)
+ $(trace-gen) $(CXX) -x c++-header -o $@ $< $(GLOBAL_CXXFLAGS)
 
 clean-files += $(GCH) $(PCH)