gen-version.pl vulnerable to dangling symlinks

[cognominal]

When editing with emacs, sometimes dangling links are left behind.
In the nqp repo, this causes the build to fail with:
Open failed: No such file or directory at tools/build/gen-version.pl line 35.
The workaround is easy: remove the dangling link

perl tools/build/gen-version.pl /Users/cog/git/nqp/install /Users/cog/git/nqp/install/share/nqp/lib > gen/moar/stage1/nqp-config.nqp
Open failed: No such file or directory at tools/build/gen-version.pl line 35.
make: *** [gen/moar/stage1/nqp.moarvm] Error 2