Remove non-Windows includes from Makefile

src/Makefile

@@ -339,9 +339,12 @@ endif
 
 # Define include paths for required headers
 # NOTE: Several external required libraries (stb and others)
-INCLUDE_PATHS = -I. -Iexternal -Iexternal/glfw/include
+INCLUDE_PATHS = -I. -Iexternal/glfw/include
 
 ifeq ($(PLATFORM),PLATFORM_DESKTOP)
+    ifeq ($(PLATFORM_OS),WINDOWS)
+        INCLUDE_PATHS += -Iexternal
+    endif
     ifeq ($(PLATFORM_OS),BSD)
         INCLUDE_PATHS += -I/usr/local/include
         LDFLAGS += -L. -Lsrc -L/usr/local/lib -L$(RAYLIB_RELEASE_PATH)