fix(build): Flag transitive dependency on Git if *not* using system LuaRocks

The lua zlib rock requires Git to fetch sources, hence we need it for
vendoring. Sadly the error from LuaRocks isn't getting propogated
resulting in an apparently successfull but incomplete install.

Author: alerque

configure.ac

@@ -217,6 +217,7 @@ AM_COND_IF([SYSTEM_LUAROCKS], [
     AX_LUA_MODULE([vstruct], [vstruct])
 ], [
     QUE_PROGVAR([luarocks])
+    QUE_PROGVAR([git]) # required for luarocks to install zlib rock
 ])
 
 QUE_FONT(Gentium Plus)