build: disable reduced exports by default

This is really a packager's option. While it's helpful to encourage devs to
test this option for daily builds, it's not reliable in several real-world
use-cases. Some older libstdc++ runtimes (freebsd 9, debian wheezy, for
example) fail to properly catch exceptions due to mismatched type_info.

See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=19664 for more info.