events: Let arbitrary signals to simulate iOS/Android backgrounding events.

This lets you build a custom embedded device that roughly offers the "this
process is going to the background NOW" semantics of SDL on a mobile device.

CMakeLists.txt

@@ -341,6 +341,8 @@ set_option(VIDEO_VIVANTE       "Use Vivante EGL video driver" ${UNIX_SYS})
 dep_option(VIDEO_VULKAN        "Enable Vulkan support" ON "ANDROID OR APPLE OR LINUX OR WINDOWS" OFF)
 set_option(VIDEO_KMSDRM        "Use KMS DRM video driver" ${UNIX_SYS})
 dep_option(KMSDRM_SHARED       "Dynamically load KMS DRM support" ON "VIDEO_KMSDRM" OFF)
+option_string(BACKGROUNDING_SIGNAL "number to use for magic backgrounding signal or 'OFF'" "OFF")
+option_string(FOREGROUNDING_SIGNAL "number to use for magic foregrounding signal or 'OFF'" "OFF")
 
 # TODO: We should (should we?) respect cmake's ${BUILD_SHARED_LIBS} flag here
 # The options below are for compatibility to configure's default behaviour.
@@ -384,6 +386,14 @@ else()
 endif()
 set(HAVE_ASSERTIONS ${ASSERTIONS})
 
+if(NOT BACKGROUNDING_SIGNAL STREQUAL "OFF")
+  add_definitions("-DSDL_BACKGROUNDING_SIGNAL=${BACKGROUNDING_SIGNAL}")
+endif()
+
+if(NOT FOREGROUNDING_SIGNAL STREQUAL "OFF")
+  add_definitions("-DSDL_FOREGROUNDING_SIGNAL=${FOREGROUNDING_SIGNAL}")
+endif()
+
 # Compiler option evaluation
 if(USE_GCC OR USE_CLANG)
   # Check for -Wall first, so later things can override pieces of it.