rockspec: make build_command more robust
rockspec: make build_command more robust

ardje/luaposix,luaposix/luaposix,snegovick/luaposix,jimmyharris/luaposix,ardje/luaposix,jimmyharris/luaposix,rrthomas/luaposix,rrthomas/luaposix,snegovick/luaposix,zevv/luaposix,zevv/luaposix