Change -std=gnu++11 to -std=c++11

I propose changing the default `-std=` compiler flag from the non-
standard `gnu++11` to `c++11`. Our code is fully C++11 compliant and
that should be reflected in the C++ standard we choose as default.

Furthermore there's an ambiguity as we make people use
`-DCMAKE_CXX_FLAGS="-std=c++11"` when this is already handled
(differently) in `CMakeLists.txt`. See the [pixls.us](https://discuss.pixls.us/t/rawtherapee-5-and-dcmake-cxx-flags/3145/3)
discussion. You also see it in `AboutThisBuild.txt`:

```
Build flags: -std=c++11 -Wno-deprecated-declarations -Wno-unused-result -std=gnu++11 -march=native -Werror=unused-label -fopenmp -Werror=unknown-pragmas -O3 -DNDEBUG
```

This commit changes `-std=gnu++11` to `-std=c++11` and builds fine
without `-DCMAKE_CXX_FLAGS="-std=c++11"`.

CMakeLists.txt

@@ -21,8 +21,8 @@ endif ()
 string (TOUPPER ${CMAKE_BUILD_TYPE} UPPER_CMAKE_BUILD_TYPE)
 
 # Set required C and C++ standards and check GCC version
-SET (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=gnu11")
-SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
+SET (CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -std=c11")
+SET (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS "4.9")
     message(FATAL_ERROR "Building RawTherapee requires using GCC version 4.9 or higher!")