Get --enable-gcc-warnings to work after touching configure.ac.

Preserve ACLOCAL_PATH in later builds, so that by default it has
the same value as it did in the first build after initial checkout.
* Makefile.in (ACLOCAL_PATH): New macro.
($(srcdir)/aclocal.m4): Use it.
* configure.ac (ACLOCAL_PATH): AC_SUBST it.
* autogen.sh (env_space): New var.
Tell user what variables, if any, to pass to 'configure'.