CMake: iOS support added

When using a recent version of CMake (3.14+), this should make it possible to:
- build SDL for iOS, both static and dynamic
- build SDL test apps (as iOS .app bundles)
- generate a working SDL_config.h for iOS (using SDL_config.h.cmake as a basis)

To use, set the following CMake variables when running CMake's configuration stage:
- CMAKE_SYSTEM_NAME=iOS
- CMAKE_OSX_SYSROOT=<SDK>  (examples: iphoneos, iphonesimulator, iphoneos12.4, /full/path/to/iPhoneOS.sdk, etc.)
- CMAKE_OSX_ARCHITECTURES=<semicolon-separated list of CPU architectures> (example: "arm64;armv7s")

Examples:
- for Simulator, using the latest, installed SDK:
    cmake path/to/SDL -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_SYSROOT=iphonesimulator -DCMAKE_OSX_ARCHITECTURES=x86_64

- for Device, using the latest, installed SDK, 64-bit only
    cmake path/to/SDL -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_SYSROOT=iphoneos -DCMAKE_OSX_ARCHITECTURES=arm64

- for Device, using the latest, installed SDK, mixed 32/64 bit
    cmake path/to/SDL -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_SYSROOT=iphoneos -DCMAKE_OSX_ARCHITECTURES="arm64;armv7s"

- for Device, using a specific SDK revision (iOS 12.4, in this example):
    cmake path/to/SDL -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_SYSROOT=iphoneos12.4 -DCMAKE_OSX_ARCHITECTURES=arm64

- for Simulator, using the latest, installed SDK, and building SDL test apps (as .app bundles):
    cmake path/to/SDL -DSDL_TEST=1 -DCMAKE_SYSTEM_NAME=iOS -DCMAKE_OSX_SYSROOT=iphonesimulator -DCMAKE_OSX_ARCHITECTURES=x86_64

--HG--
extra : rebase_source : 0c6beeec1886332c685dad6c665085ed4e7f2a3e

include/SDL_config.h.cmake

@@ -298,6 +298,7 @@
 
 /* Enable various sensor drivers */
 #cmakedefine SDL_SENSOR_ANDROID @SDL_SENSOR_ANDROID@
+#cmakedefine SDL_SENSOR_COREMOTION @SDL_SENSOR_COREMOTION@
 #cmakedefine SDL_SENSOR_DUMMY @SDL_SENSOR_DUMMY@
 
 /* Enable various shared object loading systems */
@@ -323,6 +324,7 @@
 #cmakedefine SDL_VIDEO_DRIVER_ANDROID @SDL_VIDEO_DRIVER_ANDROID@
 #cmakedefine SDL_VIDEO_DRIVER_HAIKU @SDL_VIDEO_DRIVER_HAIKU@
 #cmakedefine SDL_VIDEO_DRIVER_COCOA @SDL_VIDEO_DRIVER_COCOA@
+#cmakedefine SDL_VIDEO_DRIVER_UIKIT @SDL_VIDEO_DRIVER_UIKIT@
 #cmakedefine SDL_VIDEO_DRIVER_DIRECTFB @SDL_VIDEO_DRIVER_DIRECTFB@
 #cmakedefine SDL_VIDEO_DRIVER_DIRECTFB_DYNAMIC @SDL_VIDEO_DRIVER_DIRECTFB_DYNAMIC@
 #cmakedefine SDL_VIDEO_DRIVER_DUMMY @SDL_VIDEO_DRIVER_DUMMY@
@@ -396,6 +398,7 @@
 #cmakedefine SDL_POWER_LINUX @SDL_POWER_LINUX@
 #cmakedefine SDL_POWER_WINDOWS @SDL_POWER_WINDOWS@
 #cmakedefine SDL_POWER_MACOSX @SDL_POWER_MACOSX@
+#cmakedefine SDL_POWER_UIKIT @SDL_POWER_UIKIT@
 #cmakedefine SDL_POWER_HAIKU @SDL_POWER_HAIKU@
 #cmakedefine SDL_POWER_EMSCRIPTEN @SDL_POWER_EMSCRIPTEN@
 #cmakedefine SDL_POWER_HARDWIRED @SDL_POWER_HARDWIRED@
@@ -417,6 +420,9 @@
 #cmakedefine SDL_LIBSAMPLERATE_DYNAMIC @SDL_LIBSAMPLERATE_DYNAMIC@
 
 /* Platform specific definitions */
+#cmakedefine SDL_IPHONE_KEYBOARD @SDL_IPHONE_KEYBOARD@
+#cmakedefine SDL_IPHONE_LAUNCHSCREEN @SDL_IPHONE_LAUNCHSCREEN@
+
 #if !defined(__WIN32__)
 #  if !defined(_STDINT_H_) && !defined(_STDINT_H) && !defined(HAVE_STDINT_H) && !defined(_HAVE_STDINT_H)
 typedef unsigned int size_t;