Set CXX_STANDARD twice, in the toolchain and for all our sources

Hunter wants you to set the standard via a toolchain file, but some
distros may override that. See #30.

CMakeLists.txt

@@ -8,7 +8,8 @@ set(
 	FILEPATH
 	"Default toolchain"
 	)
-
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
 option(HUNTER_ENABLED "Enable Hunter package manager" OFF)
 include("cmake/HunterGate.cmake")