fix libzstd-mt underlinking issue

fix libzstd-mt underlinking issue

fix #2045
When compiling `libzstd` in multithreading mode,
the `libzstd-mt` recipe would not include `-pthread`,
resulting in an underlinked dynamic library.

Added a test on Travis to check that the library is fully linked.

This makes it possible, in some future release,
to build a multi-threaded `libzstd` dynamic library by default
as it would no longer impact the build script of user programs.