This makes sure that ./configure --enable-clibs does not instead
*disable* c libraries, as it did before. This is important for
packagers as the usual semantics is for --enable/--disable to both
work and do the right thing.

See https://www.flameeyes.eu/autotools-mythbuster/autoconf/arguments.html
for the proper semantics of the macro.

Helping distros to package the source

This fixes the warnings:

 WARNING: Failed to load plugin module 'compressd': libcompress42.so: cannot open shared object file: No such file or directory

 WARNING: Failed to load plugin module 'zlibvalid': libtinfl.so: cannot open shared object file: No such file or directory