Revert "Do not hardcode c++11 for unix builds"

This reverts commit 4348793.
It is needed for Travis CI based on Ubuntu Trusty.

Signed-off-by: Stefan Weil <[email protected]>

Author: stweil

CMakeLists.txt

@@ -131,6 +131,9 @@ if (CYGWIN)
 endif()
 
 if (UNIX)
+    # Hack for old cmake / compiler in Ubuntu Trusty.
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} --std=c++11")
+
     if (NOT ANDROID)
         set(LIB_pthread pthread)
     endif()