Fix thread flags on Solaris

Fix thread flags on Solaris

Also on other platforms use -pthread for compiling commands not just
linking because I noticed in the gcc(1) man page

  -pthread
      Adds support for multithreading with the pthreads library. This
      option sets flags for both the preprocessor and linker.

Removing the errno check in deps/coupling because it was a hack
(e165859c2ebc08b3a00adf4d99003c50ae9936ab) added to fix stdio problems.
Without adding -threads, errno is not thread local, and coupling was not
correctly checking the errno. It appears -mt does nothing to gcc/solaris.