specs: set package.path for in tree specl calls.

Specl 11 works well when called directly in a project root dir,
eg. `specl -freport -v1`, but only if the package path is set
appropriately in each `spec_helper.lua` or similar.
* specs/spec_helper.lua.in: Use `package.normalize` to safely
inject ./lib/?.lua into the head of package.path.

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