Implements fdk_aac decoder (#4764)

* audio_core: dsp_hle: implements fdk_aac decoder

* audio_core: dsp_hle: clean up and add comments

* audio_core: dsp_hle: move fdk include to cpp file

* audio_core: dsp_hle: detects broken fdk_aac...

... and refuses to initialize if that's the case

* audio_core: dsp_hle: fdk_aac: address comments...

... and rebase commits

* fdk_decoder: move fdk header to cpp file

CMakeLists.txt

@@ -35,6 +35,8 @@ CMAKE_DEPENDENT_OPTION(ENABLE_MF "Use Media Foundation decoder (preferred over F
 
 CMAKE_DEPENDENT_OPTION(COMPILE_WITH_DWARF "Add DWARF debugging information" ON "MINGW" OFF)
 
+CMAKE_DEPENDENT_OPTION(ENABLE_FDK "Use FDK AAC decoder" OFF "NOT ENABLE_FFMPEG_AUDIO_DECODER;NOT ENABLE_MF" OFF)
+
 if(NOT EXISTS ${PROJECT_SOURCE_DIR}/.git/hooks/pre-commit)
     message(STATUS "Copying pre-commit hook")
     file(COPY hooks/pre-commit
@@ -223,6 +225,12 @@ if (ENABLE_FFMPEG_VIDEO_DUMPER)
     add_definitions(-DENABLE_FFMPEG_VIDEO_DUMPER)
 endif()
 
+if (ENABLE_FDK)
+    find_library(FDK_AAC fdk-aac DOC "The path to fdk_aac library")
+    if(FDK_AAC STREQUAL "FDK_AAC-NOTFOUND")
+        message(FATAL_ERROR "fdk_aac library not found.")
+    endif()
+endif()
 # Platform-specific library requirements
 # ======================================