Make OPENAL optional also on non-windows platforms

There are some system where OPENAL is not available

CMakeLists.txt

@@ -33,19 +33,9 @@ if(WIN32 OR CMAKE_SYSTEM_NAME STREQUAL "Windows")
     option(WIN9X "Enable support for Windows 95/98/ME." OFF)
     option(DSOUND "Enable DirectSound audio. (deprecated)" OFF)
     option(DDRAW "Enable DirectDraw video backend. (deprecated)" OFF)
-    option(SDL1 "Enable SDL1 video backend." OFF)
-    option(SDL2 "Enable SDL2 video backend." ON)
-    option(OPENAL "Enable OpenAL audio backend." ON)
     add_feature_info(Windows9x WIN9X "Windows 95/98/ME support" OFF)
     add_feature_info(DirectSound DSOUND "DirectSound audio backend (deprecated)")
     add_feature_info(DirectDraw DDRAW "DirectDraw video backend (deprecated)")
-    add_feature_info(SDL1 SDL1 "SDL1 video backend")
-    add_feature_info(SDL2 SDL2 "SDL2 video backend")
-    add_feature_info(OpenAL OPENAL "OpenAL audio backend")
-else()
-    add_feature_info(SDL1 SDL1 "SDL1 video backend")
-    add_feature_info(SDL2 SDL2 "SDL2 video backend")
-    set(OPENAL TRUE)
 endif()
 
 if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
@@ -56,6 +46,9 @@ if(APPLE OR ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     endif()
 endif()
 
+add_feature_info(SDL1 SDL1 "SDL1 video backend")
+add_feature_info(SDL2 SDL2 "SDL2 video backend")
+add_feature_info(OpenAL OPENAL "OpenAL audio backend")
 add_feature_info(RemasterTD BUILD_REMASTERTD, "Remastered Tiberian Dawn dll")
 add_feature_info(RemasterRA BUILD_REMASTERRA "Remastered Red Alert dll")
 add_feature_info(VanillaTD BUILD_VANILLATD "Tiberian Dawn executable")