Improve ./configure search for lua

It is possible that the "lua" in pkg-config is not Lua 5.1.
Fortunately, pkg-config handles predicates such as "lua >= 5.1 lua <
5.2" properly.

Also, adjust pkg.m4 to print the pkg-config module being checked, rather
than the variable being defined. Otherwise ./configure says "checking
for LUA... no / checking for LUA... yes", which is less than helpful.

Change-Id: I330f42edefb1000bc381070f5232919cd9bef692