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 >[email protected] \
+   || rm -f [email protected]
+   $(AM_V_at)if test -f [email protected]; then \
+       if ! diff $@ [email protected] > /dev/null; then \
+           mv [email protected] $@; \
+       else \
+           rm [email protected]; \
+       fi \
+   fi
 package_revision.h: package_revision_raw.txt
    $(AM_V_GEN)if test -f $<; then \
      echo "#define REVISION \"`cat $<`\"" > $@; \