[common-dylan] Use realpath on OS X with _NSGetExecutablePath.

This makes OS X use realpath on the result from _NSGetExecutablePath
so that the resulting application name is the real name, rather than
potentially a symlink. This allows the compiler to be installed as
a symlink to the actual installation rather than needing wrapper
scripts.
Merge pull request #698 from waywardmonkeys/realpath-os-x

[common-dylan] Use realpath on OS X with _NSGetExecutablePath.

Good riddance.