Avoid unnecessary recompilation after ./configure (helps #3228)

We cache the old versions of files generated by configure, so that if
configure touches the file without changing it, we can detect that and
restore the timestamp.

Makefile

@@ -51,10 +51,39 @@ include mk/custom-settings.mk
 # No need to update makefiles for these targets:
 REALGOALS=$(filter-out bootstrapping-files framework-pkg clean clean_% distclean maintainer-clean show help,$(MAKECMDGOALS))
 
+# configure touches certain files even if they haven't changed.  This
+# can mean a lot of unnecessary recompilation after a re-configure, so
+# here we cache the old versions of these files so we can restore the
+# timestamps.
+#
+define check-configure-file
+# $1 = file
+if ! test -f $1.old; then \
+  echo "backing up $1"; \
+  cp $1 $1.old; \
+  touch -r $1 $1.old; \
+else \
+  if test $1 -nt $1.old; then \
+    if cmp $1 $1.old; then \
+       echo "$1 has been touched, but has not changed"; \
+       touch -r $1.old $1; \
+    else \
+       echo "$1 has changed"; \
+       cp $1 $1.old; \
+       touch -r $1 $1.old; \
+    fi \
+  fi \
+fi
+endef
+
 # NB. not the same as saying '%: ...', which doesn't do the right thing:
 # it does nothing if we specify a target that already exists.
 .PHONY: $(REALGOALS)
 $(REALGOALS) all:
+	@$(call check-configure-file,mk/config.mk)
+	@$(call check-configure-file,mk/project.mk)
+	@$(call check-configure-file,compiler/ghc.cabal)
+
 	@echo "===--- updating makefiles phase 0"
 	$(MAKE) -r --no-print-directory -f ghc.mk phase=0 just-makefiles
 ifneq "$(OMIT_PHASE_1)" "YES"

ghc.mk

@@ -922,8 +922,10 @@ endif
 distclean : clean
 	"$(RM)" $(RM_OPTS) config.cache config.status config.log mk/config.h mk/stamp-h
 	"$(RM)" $(RM_OPTS) mk/config.mk mk/are-validating.mk mk/project.mk
+	"$(RM)" $(RM_OPTS) mk/config.mk.old mk/project.mk.old
 	"$(RM)" $(RM_OPTS) extra-gcc-opts docs/users_guide/ug-book.xml
-	"$(RM)" $(RM_OPTS) compiler/ghc.cabal ghc/ghc-bin.cabal
+	"$(RM)" $(RM_OPTS) compiler/ghc.cabal compiler/ghc.cabal.old
+	"$(RM)" $(RM_OPTS) ghc/ghc-bin.cabal
 	"$(RM)" $(RM_OPTS) libraries/base/include/HsBaseConfig.h
 	"$(RM)" $(RM_OPTS) libraries/directory/include/HsDirectoryConfig.h
 	"$(RM)" $(RM_OPTS) libraries/process/include/HsProcessConfig.h