Set -std=c++11 the CMake way (#293)

CMakeLists.txt

@@ -54,11 +54,11 @@ if(DIST)
   set(sodium_USE_STATIC_LIBS ON)
 endif()
 
-set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD 11)
 set(CMAKE_CXX_EXTENSIONS OFF)
-set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
-set(CMAKE_THREAD_PREFER_PTHREAD TRUE)
-set(THREADS_PREFER_PTHREAD_FLAG TRUE)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_THREAD_PREFER_PTHREAD ON)
+set(THREADS_PREFER_PTHREAD_FLAG ON)
 
 find_package(Threads REQUIRED)
 find_package(sodium REQUIRED)
@@ -290,12 +290,7 @@ if(CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
   target_compile_options(devilution PRIVATE "/W0")
 endif()
 
-if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
-endif()
-
 if(CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
   # Style issues
   target_compile_options(devilutionx PRIVATE -Wno-parentheses -Wno-logical-op-parentheses -Wno-bitwise-op-parentheses)
   # Silence warnings about __int64 alignment hack not always being applicable