fail configure on --enable-shared if -Bsymbolic-functions doesn't work

fail configure on --enable-shared if -Bsymbolic-functions doesn't work

previously, a warning was issued in this case no matter what, even if
--disable-shared was used. now, the default for --enable-shared is
changed from "yes" to "auto", and the warning is issued by default,
but becomes an error if --enable-shared is used, and the test is
suppressed completely if --disable-shared is used.