VWR-24254: Add support for using ld.gold on linux.

To use ld.gold configure with:
  -DCMAKE_EXE_LINKER_FLAGS:STRING="-Wl,-use-gold".
ld.gold links the viewer on my machine in 8 seconds, as
opposed to 19 seconds with ld.bfd. Moreover, it uses a
LOT less memory during linking (about 750 MB instead of
2.5 GB!).

VWR-24254: Don't link with fontconfig on non-linux.

While we already added fontconfig in the above patch,
that code turned out to also be used by Windows and
Darwin (contrary to the comments in the code).
After looking at the history of commits and a
discussion on IRC it was decided that the original
coder (Kyle Ambroff <ambroff@lindenlab.com>) really
meant (LINUX AND VIEWER) instead of (NOT LINUX OR VIEWER).