Some packages, specifically Lua, do not have a standard name in their pkg-config. Lua 5.1 could be one of lua51, lua5.1, lua-5.1, or lua. It would be nice to have an "or" feature in pkgconfig-depends so I could write that as:
pkgconfig-depends: lua51 || lua5.1 || lua-5.1 || (lua >= 5.1, lua < 5.2)