Where available, use _NSGetExecutablePath() to make $^X absolute.

Where available, use _NSGetExecutablePath() to make $^X absolute.

In Configure, check whether _NSGetExecutablePath() can be used to find the
absolute pathname of the executable. If so, set usensgetexecutablepath in
config.sh and USE_NSGETEXECUTABLEPATH in config.h. If this is set, then use
this approach in S_set_caret_X() to canonicalise $^X as an absolute
path. This approach works on OS X, and possible on other platforms that
use dyld.