Replace NO_CROSS_DIRECTORY_HARDLINKS and NO_INSTALL_HARDLINKS with
INSTALL_SYMLINKS. The result is slightly improved; all symlinks will
point directly to the target rather than via multiple levels of
symlinks.
The rationale was covered in slightly more detail in d56cfc6 ("Use
symlinks instead of hardlinks for installed binaries", 2018-03-15).
Adjust the dangling-relative-symlink filter in the rpmlint config for
the new target of the git-difftool symlink.
Also drop DESTDIR and INSTALL from config.mak; they are both handled via
%make_install.
Remove the rpmlint filter for %buildroot usage which was only needed due
to DESTDIR's use in config.mak.
