Log Message:
yquake2: fix the search paths and installation on macOS

On macOS, ld(1) defaults to the output filename for the "install path"
(LC_ID_DYLIB). This is caught by pkgsrc with `otool -L` in order to
determine REQUIRES= in mk/pkgformat/pkg/metadata.mk - but it is wrong
here, since the library is moved to libexec/yquake2 before packaging.
(Error "missing library" when checking for missing run-time search
paths in check-shlibs.mk)

While there, this also fixes the default search path for the quake2
executable.

NFCI anywhere outside macOS.

Tested on NetBSD/amd64 and macOS/amd64.