Thank you, Travis CI, for reminding me that one cannot disable a header
with inclusion guards if it doesn't exist.
(cherry picked from commit 537ea4ccf1deb7b5e00c3745ab51e82a8c0696dc)
This removes all dependencies from the GLFW test programs on the Vulkan
SDK.
It also removes support for linking the GLFW shared library (dynamic
library, DLL) against the Vulkan loader static library.