configure: Avoid overwriting existing dependency files.

ChangeLog

 2013-12-03  Niels Möller  <nisse@lysator.liu.se>
 
+	* configure.ac (dummy-dep-files): Don't overwrite any existing
+	dependency files.
+
 	* x86_64/md5-compress.asm: New file, similar to the x86 version.
 	35% speedup.
 

configure.ac

@@ -135,7 +135,10 @@ if test x$enable_dependency_tracking = xyes ; then
 
   AC_CONFIG_COMMANDS([dummy-dep-files],
   [(cd "$srcdir" && find . '(' -name '*.c' -o -name '*.cxx' ')' -print) \
- | sed 's/\.c\(xx\)\{0,1\}$//' | (while read f; do echo > "$f.o.d"; echo > "$f.po.d"; done)
+ | sed 's/\.c\(xx\)\{0,1\}$//' | (while read f; do \
+      test -f "$f.o.d" || echo > "$f.o.d"; \
+      test -f "$f.po.d" || echo > "$f.po.d"; \
+   done)
 ])
 fi