As noted in #539, the symlink check was to support compiling raylib on
VirtualBox vboxfs as no symlinks are supported there.

With me using Linux as daily driver outside VirtualBox now, this 'feature'
is most likely unused by anyone, but still annoys other users (#791).
Just revert it and be done with it. If there are really others building
raylib on vboxfs or a similarly crippled file system under Linux, they
should now step forward when their build breaks.

This reverts commit b8ca51fd01.