Properly relink libmruby.a if it has changed.

Before, if none of the moon files changed, the executable was
not relinked, and it became stale. link_directories is obsolete
and should be avoided at all costs, setting the full path to
the mruby library solves the issue.

http://cmake.3232098.n2.nabble.com/Problem-with-LINK-DIRECTORIES-td6992023.html