[CMake] Add -lpthread for CMake >= 3.1

Since CMake 3.1 the if() command no longer automatically dereferences
variables named in quoted or bracket arguments.
This fixes FS#1567.

cmake/modules/CheckCXX11Features.cmake

@@ -219,9 +219,9 @@ if(NOT STDTHREAD_LINK_FLAGS STREQUAL "")
   string(REPLACE ";" ";CMAKE_EXE_LINKER_FLAGS_" vars "${vars}")
   string(TOUPPER "${vars}" vars)
   foreach(var ${vars})
-    if(NOT ${var} STREQUAL "")
+    if(NOT var STREQUAL "")
       set(${var} "${${var}} ${STDTHREAD_LINK_FLAGS}")
-    endif(NOT ${var} STREQUAL "")
+    endif()
   endforeach(var ${vars})
 endif(NOT STDTHREAD_LINK_FLAGS STREQUAL "")