Handle pthreads correctly in the cmake build.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=374

Change-Id: I03ee53cd36b2630eac3f6114e526953596ff3504

CMakeLists.txt

@@ -285,6 +285,13 @@ if (CONFIG_UNIT_TESTS)
   setup_aom_test_targets()
 endif ()
 
+if (HAVE_PTHREAD_H AND CONFIG_MULTITHREAD)
+  find_package(Threads)
+  foreach (app_target ${AOM_APP_TARGETS})
+    target_link_libraries(${app_target} PUBLIC Threads::Threads)
+  endforeach ()
+endif ()
+
 if (XCODE)
   if (CONFIG_LIBYUV OR CONFIG_WEBM_IO)
     # The Xcode generator does not obey LINKER_LANGUAGE. Because of the issue

build/cmake/aom_configure.cmake

@@ -10,6 +10,7 @@
 ##
 include(FindGit)
 include(FindPerl)
+include(FindThreads)
 
 # Generate the user config settings. This must occur before include of
 # aom_config_defaults.cmake (because it turns every config variable into a cache