configury: make sure we build std.lua when necessary.

Using wildcard for SOURCES before std.lua has been built now that
configure no longer makes it for us, means standard make or
luarocks install after building from a fresh checkout results in
std.lua not being built or installed.
* Makefile.am (SOURCES): Add src/std.lua.

NOTE: Be careful of the merge conflict when accepting the pull request.
(See my next branch for the correct resolution if in doubt!)