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)
+            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)
 

SimGearConfig.cmake.in

@@ -10,6 +10,8 @@ find_dependency(CURL)
 set(SIMGEAR_HEADLESS @SIMGEAR_HEADLESS@)
 set(SIMGEAR_SOUND @ENABLE_SOUND@)
 set(USE_AEONWAVE @USE_AEONWAVE@)
+set(SG_SOUND_USES_OPENALSOFT @SG_SOUND_USES_OPENALSOFT@)
+
 set(ENABLE_ASAN @ENABLE_ASAN@)
 set(SIMGEAR_SHARED @SIMGEAR_SHARED@)
 
@@ -23,6 +25,21 @@ if(SG_HAVE_DDS)
     find_dependency(CycloneDDS)
 endif()
 
+
+if (SIMGEAR_SOUND)
+  # find the sound dependency
+  if (USE_AEONWAVE)
+    find_dependency(AAX)
+  elseif(SG_SOUND_USES_OPENALSOFT)
+    # must force CONFIG mode when using OpenAL soft, to avoid picking
+    # the CMake supplied FindOpenAL and hence defaults to the system
+    # framework on macOS
+    find_dependency(OpenAL CONFIG)
+  else()
+    find_dependency(OpenAL)
+  endif()
+endif()
+
 # OpenRTI support
 set(ENABLE_RTI @ENABLE_RTI@)
 if(ENABLE_RTI)

simgear/simgear_config_cmake.h.in

@@ -26,6 +26,7 @@
 #cmakedefine SYSTEM_EXPAT
 #cmakedefine ENABLE_SOUND
 #cmakedefine USE_AEONWAVE
+#cmakedefine SG_SOUND_USES_OPENALSOFT
 #cmakedefine ENABLE_SIMD
 #cmakedefine ENABLE_SIMD_CODE
 #cmakedefine ENABLE_GDAL