Use C++17 standard

Also use simpler target_compile_features.  The comment says this
requires CMake 3.8, but that is less than our minimum required CMake
version, so it should be fine.

src/CMakeLists.txt

@@ -116,14 +116,6 @@ if(USE_STEAMWEBRTC)
  add_subdirectory(external/steamwebrtc)
 endif(USE_STEAMWEBRTC)
 
-set(C99_FEATURES
- c_restrict
- c_function_prototypes)
-
-set(CXX11_FEATURES
- cxx_constexpr
- cxx_auto_type)
-
 set(GNS_COMMON_FLAGS
  -fvisibility=hidden
  -fno-strict-aliasing
@@ -264,11 +256,7 @@ macro(set_clientlib_target_properties GNS_TARGET)
  )
  endif()
 
- ## Needs CMake 3.8, then we could get rid of the workaround below it.
- #target_compile_features(${GNS_TARGET} PUBLIC c_std_99 cxx_std_11)
- if(NOT CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
- target_compile_features(${GNS_TARGET} PUBLIC ${C99_FEATURES} ${CXX11_FEATURES})
- endif()
+ target_compile_features(${GNS_TARGET} PUBLIC c_std_99 cxx_std_17)
 
  if(CMAKE_CXX_COMPILER_ID MATCHES "GNU"
  OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")