Issue 83 fix: use correct pthread linker flags with clang.

1. -lpthreads was previously added to the CMAKE_CXX_FLAGS which are
not passed to the linker thus linking would fail.
2. Clang would emit a warning about -lpthreads being added to a
build instruction with -c (compile only).

This patch fixes both of these issues by adding -lpthreads to the
linker flags (and removes them from the CXX flags).

Change-Id: I5e54de3ab7eced177aa31f311926893598af5b56