configure sometimes misses passing LDFLAGS when testing linker

[Jameson Nash]

There are a couple tests in ./configure which try to link binaries, but
which don't propagate the LDFLAGS. In some cases, the compiler doesn't work
at all without those (notably, seen on FreeBSD when trying to use new-ish
gcc versions). This patch updates configure.ac to pass along the linker
flags to tests that are trying to use the linker. I think that's the right
fix? For reference, I've also attached the diff that this causes to the
configure script via autoconf.

Thanks,
Jameson
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