Auto-detect -Bsymbolic, fixes configure on macOS

The -Bsymbolic linker option is ELF-specific, and was breaking
configure on macOS unless --disable-Bsymbolic was explicitly passed.

Switching the behavior from requiring -Bsymbolic to be available
by default to just warning and continuing on without.

Fixes GNOME/librsvg#211
3 jobs for auto-Bsymbolic in 7 minutes and 50 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #9637
fedora:test

00:07:50

 
  Lint
manual #9639
allowed to fail manual
clippy
manual #9638
allowed to fail manual
rustfmt