build: fix Lua detection

We need to manually define the flag since we are using a separate identifier
for each of the Lua checks. This was done before 9b45b48 by the composed check
with a define_key (see waftools/checks/generic.py).

The pkg-config check was the only one to not redefine a define key because Waf
already does that automatically when we call the generated function with the
same identifier as the generator function. Now if they are called with two
different arguments we will get two different definitions.

Fixes #1218