fixed configure infinite loop when lua was missing from PATH

Moved lua-in-PATH check in the configure script to trigger before resolving symlink, before it would loop infinitely with missing operand to readlink etc. if lua was missing from PATH

    .....
    checking for sha1sum... /usr/bin/sha1sum
    checking for more... /bin/more
    checking for lua... no
    readlink: missing operand
    Try `readlink --help' for more information.
    ./configure: line 2086: test: !=: unary operator expected
    readlink: missing operand
    Try `readlink --help' for more information.
    ./configure: line 2086: test: !=: unary operator expected
    readlink: missing operand
    Try `readlink --help' for more information.
    ./configure: line 2086: test: !=: unary operator expected
    readlink: missing operand
    Try `readlink --help' for more information.
    ./configure: line 2086: test: !=: unary operator expected
    ....
    ....