adding third party libs and defining some early API headers

tests/CMakeLists.txt

@@ -0,0 +1,9 @@
+add_executable(smoke smoke.c)
+target_link_libraries(smoke PRIVATE regolith)
+add_test(NAME smoke COMMAND smoke)
+
+if(TARGET SDL3::SDL3)
+  add_executable(smoke_sdl3 smoke_sdl3.c)
+  target_link_libraries(smoke_sdl3 PRIVATE SDL3::SDL3)
+  add_test(NAME smoke_sdl3 COMMAND smoke_sdl3)
+endif()