Avoid set c++11 flag if not needed in Config.cmake

Add entry to ChangeLog.

ChangeLog.md

@@ -2,6 +2,12 @@
 
 ## New Features / Critical Changes
 
+- *Configuration/General*
+ - Set flag -std=c++11 only if needed. Modern compilers set compatible flags
+ by default (gnu++14, etc).
+ (Pablo H Cerdan,
+ [#1222](https:/DGtal-team/DGtal/pull/1222))
+
 - *Geometry Package*
  - VoronoiMap, PowerMap, (Reverse)DistanceTransformation and ReducedMedialAxis
  now work on toric domains (with per-dimension periodicity specification).

cmake/CheckCPP11.cmake

@@ -15,7 +15,6 @@ IF (NOT CPP11_COMPATIBLE_BY_DEFAULT)
  COMPILE_DEFINITIONS "-std=c++11"
  OUTPUT_VARIABLE OUTPUT
  )
- SET(CPP11_COMPATIBLE_AFTER_FLAG CPP11_COMPATIBLE_AFTER_FLAG PARENT_SCOPE)
 ENDIF()
 
 SET(CPP11_COMPATIBLE CPP11_COMPATIBLE_BY_DEFAULT OR CPP11_COMPATIBLE_AFTER_FLAG)

cmake/DGtalConfig.cmake.in

@@ -71,12 +71,13 @@ IF (@COLOR_WITH_ALPHA_ARITH_DGTAL@)
  ADD_DEFINITIONS(-DCOLOR_WITH_ALPHA_ARITH)
 ENDIF(@COLOR_WITH_ALPHA_ARITH_DGTAL@)
 
-
-IF(NOT MSVC)
- set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+# Set c++11 flag only if needed.
+IF (NOT @CPP11_COMPATIBLE_BY_DEFAULT@)
+ IF (NOT MSVC)
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 ")
+ ENDIF()
 ENDIF()
 
-
 IF(@GMP_FOUND_DGTAL@)
  ADD_DEFINITIONS("-DWITH_GMP ")
  SET(WITH_GMP 1)