Support for different platforms. Platform_desktop_sdl.

cmake/EnumOption.cmake

@@ -0,0 +1,9 @@
+macro(enum_option var values description)
+  set(${var}_VALUES ${values})
+  list(GET ${var}_VALUES 0 default)
+  set(${var} "${default}" CACHE STRING "${description}")
+  set_property(CACHE ${var} PROPERTY STRINGS ${${var}_VALUES})
+  if (NOT ";${${var}_VALUES};" MATCHES ";${${var}};")
+    message(FATAL_ERROR "Unknown value ${${var}}. Only -D${var}=${${var}_VALUES} allowed.")
+  endif()
+endmacro()