Due to issue appliedAI-Initiative#5 add the c++11 compiler option again

This reverts commit 3c6c96e.

(cherry picked from commit 963e964)

CMakeLists.txt

@@ -8,7 +8,9 @@ ENDIF()
 # Check for c++11 support
 INCLUDE(CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-std=c++11" COMPILER_SUPPORTS_CXX11)
-IF(!COMPILER_SUPPORTS_CXX11)
+IF(COMPILER_SUPPORTS_CXX11)
+  add_compile_options(-std=c++11)
+ELSE()
   MESSAGE(ERROR "The compiler ${CMAKE_CXX_COMPILER} has no C++11 support. Please use a different C++ compiler.")
 ENDIF()