let it autodetect the LZMA path
(another reason to use BSD: there you simply use $ mmake show=VARNAME in the MirPorts Framework, or $ make _DISPLAY_MAKEVARS=VARNAME in mirmake but without any support from any *.mk include)
git-svn-id: svn://www.freewrt.org/trunk/freewrt@317 afb5a338-a214-0410-bd46-81f09a774fd1