gmake: bootstrap to fetch gnulib before configuring

[nwf]

Without this, the build dies with

cd /cheri/source/mainline/gmake && /cheri/source/mainline/gmake/autogen.sh
/cheri/source/mainline/gmake/autogen.sh: 1088: cannot open gnulib/gnulib-tool: No such file
Fatal error (in target gmake-aarch64): Command `/cheri/source/mainline/gmake/autogen.sh` failed with non-zero exit code 2. Working directory was /cheri/source/mainline/gmake

[arichardson]

Hmm I think this should probably be inside configure()? ideally setup() should not modify the source dir.

[nwf]

Even more ideally that'd work in the build dir, but it does not, so source dir modification in configure() it is.