* sl/autoconf:
build: reconfigure automatically if configure.ac changes
build: "make clean" should not remove configure-generated files
autoconf: use AC_CONFIG_COMMANDS instead of ad-hoc 'config.mak.append'
autoconf: remove few redundant semicolons
autoconf: remove some redundant shell indirections
autoconf: GIT_CONF_APPEND_LINE -> GIT_CONF_SUBST
autoconf: GIT_CONF_APPEND_LINE: change signature