skip vendored openal

Index: lib/Tempest/Engine/CMakeLists.txt
--- lib/Tempest/Engine/CMakeLists.txt.orig
+++ lib/Tempest/Engine/CMakeLists.txt
@@ -89,7 +89,10 @@ add_definitions(-DSTB_TRUETYPE_IMPLEMENTATION)
 add_definitions(-DSTB_IMAGE_WRITE_IMPLEMENTATION)
 
 ### OpenAl
-if(TEMPEST_BUILD_AUDIO)
+if(${CMAKE_SYSTEM_NAME} MATCHES "OpenBSD")
+  add_definitions(-DTEMPEST_BUILD_AUDIO)
+  add_definitions(-DAL_ALEXT_PROTOTYPES)
+elseif(TEMPEST_BUILD_AUDIO)
   add_definitions(-DTEMPEST_BUILD_AUDIO)
   if(${CMAKE_SYSTEM_NAME} MATCHES "FreeBSD")
       set(ALSOFT_BACKEND_WAVE     ON  CACHE INTERNAL "")
