libxl, buildmakevars2file: Do not relink libxl each time

Make Config.mk's definition of buildmakevars2file and
tools/libxl/Makefile's rule for _libxl_paths.h generate their files to
temporary files and only rename them into place if the intended file
has changed.  This gets rid of unnecessarily compilation and link
steps.

Add *.tmp to .hgignore.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Keir Fraser <keir.fraser@citrix.com>

.hgignore

@@ -11,6 +11,7 @@
 .*\.tar\.bz2$
 .*\.tar\.gz$
 .*~$
+.*\.tmp$
 .*\.flc$
 .*\.orig$
 .*\.rej$

Config.mk

@@ -120,16 +120,17 @@ buildmakevars2file = $(eval $(call buildmakevars2file-closure,$(1)))
 define buildmakevars2file-closure
     .PHONY: genpath
     genpath:
-	rm -f $(1);                                                    \
-	echo "SBINDIR=\"$(SBINDIR)\"" >> $(1);                         \
-	echo "BINDIR=\"$(BINDIR)\"" >> $(1);                           \
-	echo "LIBEXEC=\"$(LIBEXEC)\"" >> $(1);                         \
-	echo "LIBDIR=\"$(LIBDIR)\"" >> $(1);                           \
-	echo "SHAREDIR=\"$(SHAREDIR)\"" >> $(1);                       \
-	echo "PRIVATE_BINDIR=\"$(PRIVATE_BINDIR)\"" >> $(1);           \
-	echo "XENFIRMWAREDIR=\"$(XENFIRMWAREDIR)\"" >> $(1);           \
-	echo "XEN_CONFIG_DIR=\"$(XEN_CONFIG_DIR)\"" >> $(1);           \
-	echo "XEN_SCRIPT_DIR=\"$(XEN_SCRIPT_DIR)\"" >> $(1)
+	rm -f $(1).tmp;                                                    \
+	echo "SBINDIR=\"$(SBINDIR)\"" >> $(1).tmp;                         \
+	echo "BINDIR=\"$(BINDIR)\"" >> $(1).tmp;                           \
+	echo "LIBEXEC=\"$(LIBEXEC)\"" >> $(1).tmp;                         \
+	echo "LIBDIR=\"$(LIBDIR)\"" >> $(1).tmp;                           \
+	echo "SHAREDIR=\"$(SHAREDIR)\"" >> $(1).tmp;                       \
+	echo "PRIVATE_BINDIR=\"$(PRIVATE_BINDIR)\"" >> $(1).tmp;           \
+	echo "XENFIRMWAREDIR=\"$(XENFIRMWAREDIR)\"" >> $(1).tmp;           \
+	echo "XEN_CONFIG_DIR=\"$(XEN_CONFIG_DIR)\"" >> $(1).tmp;           \
+	echo "XEN_SCRIPT_DIR=\"$(XEN_SCRIPT_DIR)\"" >> $(1).tmp;           \
+	if ! cmp $(1).tmp $(1); then mv -f $(1).tmp $(1); fi
 endef
 
 ifeq ($(debug),y)

tools/libxl/Makefile

@@ -52,12 +52,12 @@ $(LIBXLU_OBJS): $(AUTOINCS)
 	@rm -f $*.[ch]
 	$(FLEX) --header-file=$*.h --outfile=$*.c $<
 
-genpath-target = $(call buildmakevars2file,_libxl_paths.h)
+genpath-target = $(call buildmakevars2file,_libxl_paths.h.tmp)
 $(eval $(genpath-target))
 
 _libxl_paths.h: genpath
-	sed -e "s/\([^=]*\)=\(.*\)/#define \1 \2/g" $@ >_$@
-	mv _$@ $@
+	sed -e "s/\([^=]*\)=\(.*\)/#define \1 \2/g" $@.tmp >$@.2.tmp
+	if ! cmp $@.2.tmp $@; then mv -f $@.2.tmp $@; fi
 
 libxl_paths.c: _libxl_paths.h