libc_r versus libpthread and linker flags

[walt]

I think I've found the reason that pkgsrc/boost-libs isn't working
properly on DFly, but I don't know how to fix it.

Not long ago Matt made /usr/lib/libpthread.so.0 a symlink to enable
an easy switch between libc_r and libpthread.

On my machine I have the symlink pointing to libc_r, which must be
the default, I assume.

Now, the problem with boost-libs on DFly is that the boost libs
are being linked with -pthread, which seems to be the wrong choice
for libc_r, am I right about this?  The subject has always been
confusing to me.

Could it be that the boost-libs configure script is seeing the
libpthread.so stub library and wrongly concluding that -pthread
is the correct flag?  That's seems the likely reason, but I'm
not sure.

Any thoughts?