* Remove the autoconf path overrides from configure.in

* Remove the autoconf path overrides from configure.in
* Update icons and errors to install to $(sharedstatedir)/squid/icons
and ... errors. This could be simply $(sharedstatedir)/icons and errors,
but I don't think there is much call for reusing the squid specific
stuff :}. The shareing is for NFS installs and the like via the
autoconf site override capability.
* ERR_LANGUAGE and --enable-err-language to ERR_DEFAULT_LANGUAGE to
allow choice of which of the installed languages is to be used, without
preventing the install of all languages.
* New option --enable-err-languages to choose specific languages to
install.
* blindly overwrite any existing error files, if users have custom files
they should place them in a directory of their own.