Fix Vulkan header macro not breaking compilation

Fixes #823.

README.md

@@ -97,6 +97,8 @@ information on what to include when reporting a bug.
    loader linked statically into the application
  - Bugfix: Single compilation unit builds failed due to naming conflicts (#783)
  - Bugfix: The range checks for `glfwSetCursorPos` used the wrong minimum (#773)
+ - Bugfix: Defining `GLFW_INCLUDE_VULKAN` when compiling the library did not
+           fail with the expected error message (#823)
  - [Win32] Bugfix: `glfwSetClipboardString` created an unnecessary intermediate
                    copy of the string
  - [Win32] Bugfix: Examples failed to build on Visual C++ 2010 due to C99 in

src/internal.h

@@ -40,6 +40,7 @@
     defined(GLFW_INCLUDE_NONE)      || \
     defined(GLFW_INCLUDE_GLEXT)     || \
     defined(GLFW_INCLUDE_GLU)       || \
+    defined(GLFW_INCLUDE_VULKAN)    || \
     defined(GLFW_DLL)
  #error "You must not define any header option macros when compiling GLFW"
 #endif