build adjustments: CURL_HIDDEN_SYMBOLS no longer defined in config files

configure script now provides conditional definitions for Makefile.am
that result in CURL_HIDDEN_SYMBOLS being defined by resulting makefiles
when appropriate.

Additionally, configure script option for symbol hiding control is now
named --enable-symbol-hiding --disable-symbol-hiding. While still valid,
old option name --enable-hidden-symbols --disable-hidden-symbols will
be deprecated in some future release.