build: fix semantics of AC_ARG_ENABLE (issue #63).

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.