Make GLFW_VULKAN_STATIC available only on Apple platforms

The Vulkan-Loader restricted support for static linking to only
Apple platforms.

CMakeLists.txt

@@ -27,7 +27,10 @@ option(GLFW_BUILD_EXAMPLES "Build the GLFW example programs" ${GLFW_STANDALONE})
 option(GLFW_BUILD_TESTS "Build the GLFW test programs" ${GLFW_STANDALONE})
 option(GLFW_BUILD_DOCS "Build the GLFW documentation" ON)
 option(GLFW_INSTALL "Generate installation target" ON)
+
+if(APPLE)
 option(GLFW_VULKAN_STATIC "Assume the Vulkan loader is linked with the application" OFF)
+endif()
 
 include(GNUInstallDirs)
 include(CMakeDependentOption)

README.md

@@ -130,6 +130,7 @@ information on what to include when reporting a bug.
  - Made joystick subsystem initialize at first use (#1284,#1646)
  - Updated the minimum required CMake version to 3.1
  - Disabled tests and examples by default when built as a CMake subdirectory
+ - Make the `GLFW_VULKAN_STATIC` option available only on Apple platforms
  - Bugfix: The CMake config-file package used an absolute path and was not
    relocatable (#1470)
  - Bugfix: Video modes with a duplicate screen area were discarded (#1555,#1556)

docs/compile.dox

@@ -225,7 +225,7 @@ with the library.
 
 @anchor GLFW_VULKAN_STATIC
 __GLFW_VULKAN_STATIC__ determines whether to use the Vulkan loader linked
-directly with the application.
+directly with the application. This is only available on Apple platforms.
 
 
 @subsubsection compile_options_win32 Windows specific CMake options
@@ -271,7 +271,7 @@ must also define @b _GLFW_BUILD_DLL.  Otherwise, you must not define it.
 
 If you are linking the Vulkan loader directly with your application then you
 must also define @b _GLFW_VULKAN_STATIC.  Otherwise, GLFW will attempt to use the
-external version.
+external version. This is only available on Apple platforms.
 
 If you are using a custom name for the Vulkan, EGL, GLX, OSMesa, OpenGL, GLESv1
 or GLESv2 library, you can override the default names by defining those you need

docs/vulkan.dox

@@ -31,16 +31,16 @@ are also guides for the other areas of the GLFW API.
 
 @section vulkan_loader Linking against the Vulkan loader
 
-By default, GLFW will look for the Vulkan loader on demand at runtime via its
+GLFW looks for the Vulkan loader on demand at runtime via its
 standard name (`vulkan-1.dll` on Windows, `libvulkan.so.1` on Linux and other
 Unix-like systems and `libvulkan.1.dylib` on macOS).  This means that GLFW does
-not need to be linked against the loader.  However, it also means that if you
-are using the static library form of the Vulkan loader GLFW will either fail to
-find it or (worse) use the wrong one.
+not need to be linked against the loader.
 
-The @ref GLFW_VULKAN_STATIC CMake option makes GLFW call the Vulkan loader
-directly instead of dynamically loading it at runtime.  Not linking against the
-Vulkan loader will then be a compile-time error.
+@macos On Apple platforms it is possible to link to the library form of the Vulkan
+Loader using the @ref GLFW_VULKAN_STATIC CMake option. Not linking against the
+Vulkan loader will then be a compile-time error. If you are using the static
+library form of the Vulkan Loader but do not enable @ref GLFW_VULKAN_STATIC,
+GLFW will either fail to find it or (worse) use the wrong one.
 
 @macos Because the Vulkan loader and ICD are not installed globally on macOS,
 you need to set up the application bundle according to the LunarG SDK