Add C++11 as build requirement in CMake

[uilianries]

The Kealib uses C++11 for instance:

• kealib/src/libkea/KEAAttributeTable.cpp

  Line 186 in 659d893

  auto iterField = fields->find(name);

• kealib/src/libkea/KEAImageIO.cpp

  Line 3102 in 659d893

  H5::H5File *keaImgH5File = nullptr;

However, the CMakeLists.txt does not configures, which breaks in older compiler that does not C++11 or higher as defaulted standard version, like GCC 5:

cmake -G "Unix Makefiles" -DCMAKE_TOOLCHAIN_FILE="build/Release/generators/toolchain.cmake" -DCMAKE_INSTALL_PREFIX="package" -DHDF5_USE_STATIC_LIBRARIES="ON" -DHDF5_PREFER_PARALLEL="OFF" -DHDF5_THREADSAFE="OFF" -DLIBKEA_WITH_GDAL="OFF" -DCMAKE_POLICY_DEFAULT_CMP0091="NEW" -DCMAKE_BUILD_TYPE="Release" ${PWD}
-- The C compiler identification is GNU 5.4.0
-- The CXX compiler identification is GNU 5.4.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/gcc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
...
-- Generating done

cmake --build build/Release -- -j12
Scanning dependencies of target kea
[ 14%] Building CXX object src/CMakeFiles/kea.dir/libkea/KEAImageIO.cpp.o
[ 28%] Building CXX object src/CMakeFiles/kea.dir/libkea/KEAAttributeTableInMem.cpp.o
[ 42%] Building CXX object src/CMakeFiles/kea.dir/libkea/KEAAttributeTable.cpp.o
[ 57%] Building CXX object src/CMakeFiles/kea.dir/libkea/KEAAttributeTableFile.cpp.o
In file included from /usr/include/c++/5/cinttypes:35:0,
                 from /home/foobar/kealib/src/libkea/KEAAttributeTable.cpp:31:
/usr/include/c++/5/bits/c++0x_warning.h:32:2: error: #error This file requires compiler and library support for the ISO C++ 2011 standard. This support must be enabled with the -std=c++11 or -std=gnu++11 compiler options.
 #error This file requires compiler and library support \
  ^

This PR adds explicit requirement for C++11 in the CMakeLists.txt.

[gillins]

Thanks @uilianries !