do-not-recompile

diff --git a/Makefile.am b/Makefile.am
--- a/Makefile.am
+++ b/Makefile.am
@@ -91,8 +91,15 @@
 # successfully; the rm -f ensures both
 package_revision_raw.txt:
    $(AM_V_GEN)REAL_BLDDIR=$$PWD/$(top_builddir); \
-   (hg --cwd $(srcdir) id -i --debug) 2>/dev/null >$@ \
-   || rm -f $@
+   (hg --cwd $(srcdir) id -i --debug) 2>/dev/null >$@.new \
+   || rm -f $@.new
+   $(AM_V_at)if test -f $@.new; then \
+       if ! diff $@ $@.new > /dev/null; then \
+           mv $@.new $@; \
+       else \
+           rm $@.new; \
+       fi \
+   fi
 package_revision.h: package_revision_raw.txt
    $(AM_V_GEN)if test -f $<; then \
      echo "#define REVISION \"`cat $<`\"" > $@; \