configure: Use the full path to luac to support our contrib system.
configure: Use the full path to luac to support our contrib system.

This fixes the build on Mac OS X. This is probably not the best way
for non-contrib dependant build, but I don't have any better idea.
Feel free to make it works differently.

shyamalschandra/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,shyamalschandra/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,krichter722/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc,krichter722/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,xkfz007/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.2,shyamalschandra/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1