cmake: add SDL2 support to cmake build script

src/CMakeLists.txt

@@ -1,10 +1,3 @@
-if(LIBRW_PLATFORM_GL3)
-    set(OpenGL_GL_PREFERENCE GLVND)
-    find_package(OpenGL REQUIRED)
-    find_package(GLEW REQUIRED)
-    find_package(glfw3 REQUIRED)
-endif()
-
 add_library(librw
     "${PROJECT_SOURCE_DIR}/args.h"
     "${PROJECT_SOURCE_DIR}/rw.h"
@@ -145,12 +138,33 @@ set_target_properties(librw
     )
 
 if(LIBRW_PLATFORM_GL3)
+
+
+if(LIBRW_PLATFORM_GL3)
+    set(OpenGL_GL_PREFERENCE GLVND)
+    find_package(OpenGL REQUIRED)
+    find_package(GLEW REQUIRED)
+
     target_link_libraries(librw
         PUBLIC
-            glfw
-            GLEW::GLEW
             OpenGL::GL
+            GLEW::GLEW
         )
+    if (LIBRW_GL3_GFXLIB STREQUAL "GLFW")
+        find_package(glfw3 REQUIRED)
+        target_link_libraries(librw
+            PUBLIC
+                glfw
+            )
+    elseif (LIBRW_GL3_GFXLIB STREQUAL "SDL2")
+        find_package(SDL2 REQUIRED)
+        target_compile_definitions(librw PUBLIC LIBRW_SDL2)
+        target_link_libraries(librw
+            PUBLIC
+                SDL2::SDL2
+            )
+    endif()
+endif()
 elseif(LIBRW_PLATFORM_D3D9)
     target_link_libraries(librw
         PUBLIC