allow custom opengl loader

xmake.lua

@@ -1,6 +1,12 @@
 add_rules("mode.debug", "mode.release")
 
---add_requires("SDL2", "glfw", "zlib", "glm")
+if is_os("linux") then
+    GL_LOADER = "glad"
+    add_defines("TD_IMPL_OPENGL_LOADER_GLAD");
+    add_requires("libsdl >= 2", "zlib", GL_LOADER)
+else
+    add_defines("TD_IMPL_OPENGL_LOADER_GLBINDING2");
+end
 
 target("TowerDefense")
     set_kind("binary")
@@ -8,15 +14,14 @@ target("TowerDefense")
     add_files("src/*.cpp", "src/*/*.cpp", "src/*/*/*.cpp", "src/*/*/*/*.cpp")
     add_cxflags("-pthread")
 
+    add_links("pthread")
+
     set_languages("c++17")
-
-    add_links("glbinding", "z", "SDL2")
-
-    if is_os("linux") then
-        add_links("GL", "pthread")
-    end
+    
     if is_os("windows") then
-        add_links("opengl32", "ws2_32", "mingw32", "gdi32", "imm32")
+        add_links("opengl32", "ws2_32", "mingw32", "gdi32", "imm32", "z", "SDL2", "glbinding")
+    else
+        add_packages("zlib", "libsdl", GL_LOADER, "opengl")  
     end
     
     if is_mode("release") then