Do not generally override CXX standard.

Author: ekpyron

cmake/EthToolchains.cmake

@@ -1,5 +1,7 @@
 # Require C++17.
-set(CMAKE_CXX_STANDARD 17) # This requires at least CMake 3.8 to accept this C++17 flag.
+if (NOT DEFINED CMAKE_CXX_STANDARD)
+  set(CMAKE_CXX_STANDARD 17) # This requires at least CMake 3.8 to accept this C++17 flag.
+endif ()
 set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 set(CMAKE_CXX_EXTENSIONS OFF)