Always run mkgitver prior to a build

Some hosts were not running `mkgitver` when they should, so tweak the
script to only update the timestamp when the file's data changes and
then always run the script when performing a build.

Makefile.in

@@ -131,12 +131,12 @@ rounding.h: rounding.c rsync.h proto.h
 	fi
 	@rm -f rounding.out
 
-# While $(wildcard ...) is a GNU make idiom, at least other makes should just turn it into an
-# empty string (we need something that will vanish if we're not building a git checkout).
-# If you want an updated git version w/o GNU make, remove git-version.h after a pull.
-git-version.h: mkgitver $(wildcard $(srcdir)/.git/logs/HEAD)
+git-version.h: ALWAYS_RUN
 	$(srcdir)/mkgitver
 
+.PHONY: ALWAYS_RUN
+ALWAYS_RUN:
+
 simd-checksum-x86_64.o: simd-checksum-x86_64.cpp
 	@$(srcdir)/cmd-or-msg disable-simd $(CXX) -I. $(CXXFLAGS) $(CPPFLAGS) -c -o $@ $(srcdir)/simd-checksum-x86_64.cpp
 

mkgitver

@@ -3,12 +3,18 @@
 srcdir=`dirname $0`
 gitver=`git describe --abbrev=8 2>/dev/null`
 
+if [ ! -f git-version.h ]; then
+    touch git-version.h
+fi
+
 case "$gitver" in
     *.*)
-	echo "#define RSYNC_GITVER \"$gitver\"" >git-version.h
-	;;
-    *)
-	# We either create an empty file or update the time on what the user gave us.
-	touch git-version.h
+	echo "#define RSYNC_GITVER \"$gitver\"" >git-version.h.new
+	if ! diff git-version.h.new git-version.h >/dev/null; then
+	    echo "Updating git-version.h"
+	    mv git-version.h.new git-version.h
+	else
+	    rm git-version.h.new
+	fi
 	;;
 esac