The NetBSD sonames for X11 and related libraries is more stable than on
OpenBSD but the version numbers are still bumped more often than their
Linux counterparts, even excluding the one-time version bump across all
X11 related libraries.

This commit moves to using version-less sonames for X11 and related
libraries on NetBSD, which will hopefully be more forward-compatible
than hard-coding NetBSD-specific sonames.

This may not be the correct long-term solution but it runs now.

Binaries also appear to need an LD_LIBRARY_PATH or rpath entry of
/usr/X11R7/lib in order for the libraries to be found by dlopen.

Tested on NetBSD 9.2.