Fix meson check for lua 5.1 and lgi
For rationale see #33 (closed).
This is my first time working with meson, so if anything could be done better, let me know.
Also, I'm not sure whether this handles all the possible combinations of Lua 5.1. and LuaJIT and lgi
being available on the system or not. I never used the old build system and I don't know how it handled all these cases.
Signed-off-by: Christopher Arndt chris@chrisarndt.de