autotools: it is useless to destroy the cache everytime and request to use it.

This patch reenable configure cache and should make configure faster after a few
run.

autogen.sh

@@ -1,10 +1,32 @@
 #!/bin/sh
 
 rm -rf autom4te.cache
-rm -f aclocal.m4 ltmain.sh config.cache
+rm -f aclocal.m4 ltmain.sh
 
 autoreconf -vif
 
+W=0
+
+rm -f config.cache-env.tmp
+echo "OLD_PARM=\"$@\"" >> config.cache-env.tmp
+echo "OLD_CFLAGS=\"$CFLAGS\"" >> config.cache-env.tmp
+echo "OLD_PATH=\"$PATH\"" >> config.cache-env.tmp
+echo "OLD_PKG_CONFIG_PATH=\"$PKG_CONFIG_PATH\"" >> config.cache-env.tmp
+echo "OLD_LDFLAGS=\"$LDFLAGS\"" >> config.cache-env.tmp
+
+cmp config.cache-env.tmp config.cache-env >> /dev/null
+if [ $? -ne 0 ]; then
+   W=1;
+fi
+
+if [ $W -ne 0 ]; then
+   echo "Cleaning configure cache...";
+   rm -f config.cache config.cache-env
+   mv config.cache-env.tmp config.cache-env
+else
+   rm -f config.cache-env.tmp
+fi
+
 if [ -z "$NOCONFIGURE" ]; then
   exec ./configure -C "$@"
 fi