CMake config option to use OpenAL-soft

THis enables overriding the Apple OpenAL.framework on
macOS, and using OpenAL-soft instead.

CMakeLists.txt

@@ -96,6 +96,8 @@ option(ENABLE_GDAL      "Set to ON to build SimGear with GDAL support" OFF)
 option(ENABLE_TESTS     "Set to OFF to disable building SimGear's test applications" ON)
 option(ENABLE_SOUND     "Set to OFF to disable building SimGear's sound support" ON)
 option(USE_AEONWAVE     "Set to ON to use AeonWave instead of OpenAL" ON)
+option(USE_OPENALSOFT   "Set to ON to use OpenAL from OpenAL-soft" OFF)
+
 option(ENABLE_PKGUTIL   "Set to ON to build the sg_pkgutil application (default)" ON)
 option(ENABLE_SIMD      "Enable SSE/SSE2 support for compilers" ON)
 option(ENABLE_SIMD_CODE	"Enable SSE/SSE2 support code for compilers" OFF)
@@ -147,13 +149,26 @@ else()
 
         if(NOT AAX_FOUND)
             set(USE_AEONWAVE FALSE)
-            find_package(OpenAL REQUIRED)
+            if (USE_OPENALSOFT)
+                message(STATUS "Sound requested to use OpenAL-soft: ensure Config.cmake files are in CMAKE_PREFIX_PATH")
+               find_package(OpenAL REQUIRED CONFIG)
+               if (TARGET OpenAL::OpenAL)
+                set(SG_SOUND_USES_OPENALSOFT 1)
+               endif()
+            else()
+              # regular OpenAL is found via the CMake distro-suppling FindOpenAL
+              find_package(OpenAL REQUIRED)
+            endif()
+
+            message(STATUS "OpenAL: ${OPENAL_LIBRARY}")
         endif()
 
         if(AAX_FOUND)
             message(STATUS "Sound support: AeonWave")
+        elseif (TARGET OpenAL::OpenAL)
+           message(STATUS "Sound support: OpenAL-soft")
         else()
-            message(STATUS "Sound support: OpenAL")
+            message(STATUS "Sound support: OpenAL (from system)")
         endif()
     endif(ENABLE_SOUND)