Allow suppression of --no-fatal-warnings.

[jonathan]

Summary

When building with an external toolchain whose linker doesn't support the
--no-fatal-warnings flag, we need a mechanism to suppress it. This mechanism
takes the form of a new make variable, LD_NO_FATAL_WARNS. If it is not set,
it defaults to --Wl,--no-fatal-warnings as now. However, it can be defined
in src.conf, at the command line, etc., to be the empty string.

Sponsored by: DARPA, AFRL

[jonathan]

Of course, if we wanted to ignore the --no-fatal-warnings flag altogether, we could just do:

.if !defined(LD_FATAL_WARNINGS) || ${LD_FATAL_WARNINGS} != "no"
SOLINKOPTS+=        -Wl,---fatal-warnings
.endif

However, I wouldn't want to presume that we want to do that...

[jonathan]

This seems to have been obviated by rS303274.