MSVC: lower C4100 warning level from 4 to 3

This better aligns with OSX/Linux warnings.

Although `__pragma(warning(suppress:4100))` is not the same as
`__attribute__((__unused__))` in gcc (which only affects the attribute
instead of the line), it still seems to be better to use it than to
hide the warning entirely.

CMakeLists.txt

@@ -1631,6 +1631,7 @@ elseif(CMAKE_C_COMPILER_ID MATCHES "MSVC")
     # warning level:
     "/W3"
     "/w34062"  # switch statement contains 'default' but no 'case' labels
+    "/w34100"  # 'identifier' : unreferenced formal parameter
     "/w34115"  # 'type' : named type definition in parentheses
     "/w34189"  # local variable is initialized but not referenced
     # see https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/c5038?view=vs-2017