Use gcc-generated dependency files if we can (GNU Make and gcc required).

Use gcc-generated dependency files if we can (GNU Make and gcc required).

* Makefile.in: If AUTO_DEPEND is defined, make gcc generate
dependency files in deps/. Include those files into Makefile.

* config.in: Generated (AUTO_DEPEND).

* configure.in: --enable-autodepend is new.  Check for GNU Make
and that gcc supports -MMD -MF. Define AUTO_DEPEND if we can use
gcc and GNU make to generate dependencies.