FindVulkan: Support for finding glslc

The GLSL SPIR-V compiler is part of the Vulkan SDK and may be used
by projects for compiling shaders as part of the build process.
This is not strictly required to build a Vulkan application, which
is why the variable is not part of the REQUIRED_VARs for the module.
Status Job ID Name Coverage
  External
passed ghostflow-check-master