Add more C++-standard-related CMake options to CMakeLists.txt.

set(CMAKE_CXX_STANDARD_REQUIRED ON)
This makes C++14 a requirement and prevents a "decay" to C++98 when the
compiler does not support C++14. RetDec and related tools require C++14 to
build, so the sooner the build fails the better.

set(CMAKE_CXX_EXTENSIONS OFF)
This disables the use of compiler-specific extensions. For example, by default,
CMake passes -std=gnu++14 to GCC on Linux. We want to build with -std=c++14 to
prevent use of non-portable compiler extensions, which is what the disabling of
this option does.

For more details, see avast/retdec#76

CMakeLists.txt

@@ -10,6 +10,8 @@ if (NOT CMAKE_BUILD_TYPE)
 endif()
 
 set(CMAKE_CXX_STANDARD 14)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
 set(CMAKE_INSTALL_RPATH "${CMAKE_INSTALL_PREFIX}/lib")
 
 add_subdirectory(src)