Remove unnecessary flag in macOS build

Remove unnecessary flag in macOS build

The configuration logic for adding the `-search_paths_first` linker
flag on Darwin does not correctly handle cross compilation.  It should
check the value of $krb5_cv_host rather than `uname -s` to detect when
the compilation target is Darwin, rather than the build machine.

It turns out `-search_paths_first` has been the default behavior of ld
on macOS since XCode 4.  So just remove that bit of logic entirely.
(The flag was added in commit acd27af0e845f8b93de2e226cc2ec9ac8af52077
in 2004; XCode 4 was released in 2010.)

[ghudson@mit.edu: edited commit message]