hidapi: Add SDL_hidapi.c, allows support for multiple hidapi backends.

This is currently supported on Linux and macOS. iOS and Android are not
supported at all, Windows support could be added with some changes to the libusb
backend. The Visual Studio and Xcode projects do not use this feature.

Based on Valve Software's hid.cpp, written in collaboration with Andrew Eikum.

CMakeLists.txt

@@ -156,11 +156,10 @@ if(UNIX OR MINGW OR MSYS)
 endif()
 
 # The hidraw support doesn't catch Xbox, PS4 and Nintendo controllers,
-#  so we'll just use libusb when it's available. Except that libusb
-#  requires root permissions to open devices, so that's not generally
-#  useful, and we'll disable this by default on Unix. Windows and macOS
-#  can use it without root access, though, so enable by default there.
-if(WINDOWS OR APPLE OR ANDROID)
+#  so we'll just use libusb when it's available. libusb does not support iOS,
+#  so we default to yes on iOS.
+#  TODO: Windows can support libusb, the hid.c file just depends on Unix APIs
+if(WINDOWS OR IOS OR ANDROID)
   set(HIDAPI_SKIP_LIBUSB TRUE)
 else()
   set(HIDAPI_SKIP_LIBUSB FALSE)
@@ -169,6 +168,14 @@ if (HIDAPI_SKIP_LIBUSB)
   set(OPT_DEF_HIDAPI ON)
 endif()
 
+# On the other hand, *BSD specifically uses libusb only, so we make a special
+#  case just for them.
+if(FREEBSD OR NETBSD OR OPENBSD OR BSDI)
+  set(HIDAPI_ONLY_LIBUSB TRUE)
+else()
+  set(HIDAPI_ONLY_LIBUSB FALSE)
+endif()
+
 # Compiler info
 if(CMAKE_COMPILER_IS_GNUCC)
   set(USE_GCC TRUE)
@@ -1376,6 +1383,7 @@ elseif(WINDOWS)
 
   if(SDL_JOYSTICK)
     CheckHIDAPI()
+    # TODO: Remove this hid.c block when SDL_hidapi.c is supported on Windows!
     if(HAVE_HIDAPI)
       set(SOURCE_FILES ${SOURCE_FILES} ${SDL2_SOURCE_DIR}/src/hidapi/windows/hid.c)
     endif()
@@ -1466,8 +1474,6 @@ elseif(APPLE)
     if(HAVE_HIDAPI)
       if(IOS)
         set(SOURCE_FILES ${SOURCE_FILES} ${SDL2_SOURCE_DIR}/src/hidapi/ios/hid.m)
-      else()
-        set(SOURCE_FILES ${SOURCE_FILES} ${SDL2_SOURCE_DIR}/src/hidapi/mac/hid.c)
       endif()
     endif()
     set(SDL_JOYSTICK_IOKIT 1)