libtool support for LLVM's LLD linker

[Ed Maste]

I am (with a few others) trying to build all of the applications in
the FreeBSD ports tree using LLVM's LLD as the linker. Right now about
90% of the 26000 pieces of software build. The majority of the
failures are due to libtool not detecting LLD and passing in the
appropriate arguments, and I'd like to get some feedback before I
propose a patch.

LLD is mostly compatible with GNU ld (as is GNU gold), and we can fix
many of the failures with changes along the lines of:

 case `$LD -v 2>&1 </dev/null` in
-*GNU* | *'with BFD'*)
+*GNU* | *'with BFD'* | *LLD*)

That is, just treating LLD as GNU ld (for the BSDs and GNU/Linux).

I believe this will be the most straightforward way to handle this
case and the most maintainable on an ongoing basis. It will require
only a few modifications to existing cases, rather than entirely new
tests for LLD. If proceeding with this approach I'd propose some
additional variable name and cosmetic changes - e.g. replacing
"checking for GNU ld" with "checking for a GNU-compatible ld."

Please let me know if this seems broadly acceptable, and I'll work on a patch.