Require C++ version via cmake features

This works nicely with znc-buildmod because it will now ask cmake to
support at least C++17, but not limit modules to it

CMakeLists.txt

@@ -42,8 +42,6 @@ endfunction()
 list(APPEND CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/cmake")
 
 include(TestCXX17)
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CXX_STANDARD_REQUIRED true)
 if(NOT CYGWIN)
 	# We don't want to use -std=gnu++17 instead of -std=c++17, but among other
 	# things, -std=c++17 on cygwin defines __STRICT_ANSI__ which makes cygwin

src/CMakeLists.txt

@@ -86,16 +86,11 @@ set_target_properties(znclib PROPERTIES
 	OUTPUT_NAME "znc"
 	SOVERSION "${ZNC_VERSION}")
 
-# CMake started supporting metafeature cxx_std_11 only in 3.8
-set(required_cxx11_features
-	cxx_range_for cxx_nullptr cxx_override
-	cxx_lambdas cxx_auto_type)
-target_compile_features(znc PUBLIC ${required_cxx11_features})
-target_compile_features(znclib PUBLIC ${required_cxx11_features})
-
 add_library(ZNC INTERFACE)
 target_link_libraries(ZNC INTERFACE ${znc_link} ${zncpubdeps})
 target_compile_definitions(ZNC INTERFACE "znc_export_lib_EXPORTS")
+target_compile_features(ZNC INTERFACE cxx_std_17)
+target_compile_features(znclib PUBLIC cxx_std_17)
 
 if(HAVE_I18N)
 	add_subdirectory(po)