Add _GLFW_VULKAN_STATIC build macro

This allows clients to link GLFW with a staticly-built loader library.
This is useful in hermetic build systems that build the Khronos loader
from source, rather than depending on the LunarG SDK binaries.

Closes #820.

src/internal.h

@@ -149,11 +149,16 @@ typedef struct VkExtensionProperties
 } VkExtensionProperties;
 
 typedef void (APIENTRY * PFN_vkVoidFunction)(void);
+
+#if defined(_GLFW_VULKAN_STATIC)
+  PFN_vkVoidFunction vkGetInstanceProcAddr(VkInstance,const char*);
+  VkResult vkEnumerateInstanceExtensionProperties(const char*,uint32_t*,VkExtensionProperties*);
+#else
   typedef PFN_vkVoidFunction (APIENTRY * PFN_vkGetInstanceProcAddr)(VkInstance,const char*);
   typedef VkResult (APIENTRY * PFN_vkEnumerateInstanceExtensionProperties)(const char*,uint32_t*,VkExtensionProperties*);
-
   #define vkEnumerateInstanceExtensionProperties _glfw.vk.EnumerateInstanceExtensionProperties
   #define vkGetInstanceProcAddr _glfw.vk.GetInstanceProcAddr
+#endif
 
 #if defined(_GLFW_COCOA)
  #include "cocoa_platform.h"
@@ -445,8 +450,10 @@ struct _GLFWlibrary
         void*           handle;
         char**          extensions;
         uint32_t        extensionCount;
+        #if !defined(_GLFW_VULKAN_STATIC)
           PFN_vkEnumerateInstanceExtensionProperties EnumerateInstanceExtensionProperties;
           PFN_vkGetInstanceProcAddr GetInstanceProcAddr;
+        #endif
         GLFWbool        KHR_surface;
         GLFWbool        KHR_win32_surface;
         GLFWbool        KHR_xlib_surface;

src/vulkan.c

@@ -41,6 +41,8 @@ GLFWbool _glfwInitVulkan(void)
     VkResult err;
     VkExtensionProperties* ep;
     uint32_t i, count;
+
+#if !defined(_GLFW_VULKAN_STATIC)
 #if defined(_GLFW_WIN32)
     const char* name = "vulkan-1.dll";
 #else
@@ -79,6 +81,8 @@ GLFWbool _glfwInitVulkan(void)
         return GLFW_FALSE;
     }
 
+#endif // _GLFW_VULKAN_STATIC
+
     err = vkEnumerateInstanceExtensionProperties(NULL, &count, NULL);
     if (err)
     {