Replace GLFWuint64 with uint64_t

C99 stdint.h is provided by VS 2010 and later.  GLFW has not provided
testing or binaries for VS 2008 for several releases.

For earlier versions of VS there are third-party alternatives:

https://msinttypes.googlecode.com/svn/trunk/stdint.h
http://www.azillionmonkeys.com/qed/pstdint.h

This change does not affect the ABI.

src/input.c

@@ -660,16 +660,16 @@ GLFWAPI void glfwSetTime(double time)
     }
 
     _glfw.timerOffset = _glfwPlatformGetTimerValue() -
-        (GLFWuint64) (time * _glfwPlatformGetTimerFrequency());
+        (uint64_t) (time * _glfwPlatformGetTimerFrequency());
 }
 
-GLFWAPI GLFWuint64 glfwGetTimerValue(void)
+GLFWAPI uint64_t glfwGetTimerValue(void)
 {
     _GLFW_REQUIRE_INIT_OR_RETURN(0);
     return _glfwPlatformGetTimerValue();
 }
 
-GLFWAPI GLFWuint64 glfwGetTimerFrequency(void)
+GLFWAPI uint64_t glfwGetTimerFrequency(void)
 {
     _GLFW_REQUIRE_INIT_OR_RETURN(0);
     return _glfwPlatformGetTimerFrequency();