zstd @1.3.4: opportunistically links with libz, liblz4, liblzma

[ryandesign (Ryan Carsten Schmidt)]

When not using the universal variant, zstd will link with libz, liblz4 and liblzma if they are present:

$ port deps zstd
zstd @1.3.4_1 has no dependencies.
$ otool -L /opt/local/bin/zstd
/opt/local/bin/zstd:
	/opt/local/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.11)
	/opt/local/lib/liblzma.5.dylib (compatibility version 8.0.0, current version 8.4.0)
	/opt/local/lib/liblz4.1.dylib (compatibility version 1.0.0, current version 1.8.2)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)

The zstd port should declare dependencies on the zlib, lz4 and xz ports and make their use explicit via configure args.

Or, if the use of these libraries is not desired, the port must ensure they are not used, even if they are installed.

Whichever solution is chosen, the revision will need to be increased.