Simplify pthread + libtool workaround.

Improve upon r496818. Instead of setting both MAKE_ENV and patching ilmthread's
Makefile, just make sure PTHREAD_LIBS is set a configuration time. This is what
Debian does, and it achieves the same solution with fewer lines.

While here, add an explanation for why we need this.

PR: 236734
Approved by: mandree (maintainer)