Fix build rules in mmm-mode recipe

This is a followup on the pull request that I filed recently (https://github.com/dimitri/el-get/pull/2042).  The hack proposed in that commit was needed for avoiding a locally modified version.texi file interfering with the build process.  This has been reported (https://github.com/purcell/mmm-mode/issues/41) and fixed upstream (https://github.com/purcell/mmm-mode/commit/53524db13dfedcc104866c3cd3c62adda1232679).  This commit takes the upstream changes into account and replaces the cp/mv commands by "make maintainer-clean" before the autotools/make commands.