maint: don't distribute Makefile for luarocks git builds.

* .gitignore: Add Makefile.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
maint: add stdlib dependency to rockspec.

* specl-rockspec.lua (dependencies): Add stdlib.
(version): Bump rockspec number to 2.
(build-command): Simplify slightly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>