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.

.travis.yml

@@ -56,12 +56,14 @@ matrix:
         # DEVNULLRIGHTS : will request sudo rights to test permissions on /dev/null
         - DEVNULLRIGHTS=test make test
 
-    - name: gcc-6 + gcc-7 compilation   # ~ 3mn
+    - name: gcc-6 + gcc-7 + libzstdmt compilation   # ~ 6mn
       script:
         - make gcc6install gcc7install
         - CC=gcc-6 CFLAGS=-Werror make -j all
         - make clean
         - CC=gcc-7 CFLAGS=-Werror make -j all
+        - make clean
+        - LDFLAGS=-Wl,--no-undefined make -C lib libzstd-mt
 
     - name: gcc-8 + ASan + UBSan + Test Zstd   # ~6.5mn
       script: