externals: Add SDL 2.0.12

Since Bintray is (soon to be) no more, there needs to be a way to
acquire SDL2. Since 20.04's version is older than our minimum required
version (2.0.12), add it as an external.

externals/CMakeLists.txt

@@ -45,6 +45,12 @@ target_include_directories(microprofile INTERFACE ./microprofile)
 add_library(unicorn-headers INTERFACE)
 target_include_directories(unicorn-headers INTERFACE ./unicorn/include)
 
+# SDL2
+if (NOT SDL2_FOUND)
+    set(SDL2_FOUND YES)
+    add_subdirectory(SDL EXCLUDE_FROM_ALL)
+endif()
+
 # SoundTouch
 add_subdirectory(soundtouch)