Build breaks if lua.h is missing

[guillerodriguez]

I am using a dev machine that has Lua installed but not the dev package (so no lua.h). Despite configure not finding lua.h, pkcommon.h still wants to include it, which breaks the build:

In file included from pagekitec.c:31:0:
../../libpagekite/pkcommon.h:155:19: fatal error: lua.h: No existe el archivo o el directorio
 #  include <lua.h>
                   ^
compilation terminated.

I also tried to configure --without-lua, same results.

I looked inside pkcommon.h and found that the offending line is guarded by HAVE_LUA checks (pkcommon.h#L154). I am not sure where HAVE_LUA is defined (it is not present in config.h) but anyway I think this should be changed to check for HAVE_LUA_H instead.