maint: move maintainer rules into GNUmakefile.

* Makefile.am: Move maintainer rules from here...
* GNUmakefile.am: New file. ...to here.
Automatically autoreconf the directory if configure is missing.
string_ext: don't use math.mod, which doesn't exist in Lua 5.2.

* string_ext.lua (ordinalSuffix): Use '%' operator instead of
math.mod, which is compatible with Lua 5.1 and 5.2.
specl: sync from upstream.

* specs/lib/specl.lua: Upgrade to the latest upstream, where the
contain and match matchers are not broken!
specs: add specl specification for string_ext module.

* specs/string_ext_spec.lua: New file. Specl specs for
string_ext.
* Makefile.am (SPECS): Add specs/string_ext_spec.lua.