Merge branch 'ah/autoconf-fixes'

"./configure --with-expat=no" did not work as a way to refuse use
of the expat library on a system with the library installed, which
has been corrected.

* ah/autoconf-fixes:
  configure.ac: always save NO_ICONV to config.status
  configure.ac: don't overwrite NO_CURL option
  configure.ac: don't overwrite NO_EXPAT option