findtool: file name as a full path requires a slash

Kalle Vahlman's patch applied a while ago broke how the findtool
function searches for tools, as it would always check if "$file"
was present first, which thus made the bad assumption that a file
in the current directory would be a match.

I noticed when it found 'libtool' in the current directory but
libtoolize is not there, which confused the script.