cmake: add missing ${...} for variables inside generator expressions

src/audio_core/CMakeLists.txt

@@ -23,7 +23,7 @@ add_library(audio_core STATIC
     time_stretch.cpp
     time_stretch.h
 
-    $<$<BOOL:SDL2_FOUND>:sdl2_sink.cpp sdl2_sink.h>
+    $<$<BOOL:${SDL2_FOUND}>:sdl2_sink.cpp sdl2_sink.h>
 )
 
 create_target_directory_groups(audio_core)

src/input_common/CMakeLists.txt

@@ -8,7 +8,7 @@ add_library(input_common STATIC
     motion_emu.cpp
     motion_emu.h
 
-    $<$<BOOL:SDL2_FOUND>:sdl/sdl.cpp sdl/sdl.h>
+    $<$<BOOL:${SDL2_FOUND}>:sdl/sdl.cpp sdl/sdl.h>
 )
 
 create_target_directory_groups(input_common)