v1.1.2 Fixes to some build issues

Includes fixes for some issues found testing on some Linux systems. Also supports building a Median version (Medianx) without needing to install Cfitsio (although this cannot read or write FITS files). If users have not installed the glslc compiler, the release now includes pre-compiled .spv files and these can be used instead of building from source. (But without the glslc compiler it is not possible to experiment with changes to the GLSLC GPU code.)

• Adder: Vulkan setup diagnostics now better match those from Metal.
• Compiled GLSLC files (.spv) no longer deleted by Makefile target 'clean'.
• New Vulkan Makefile target 'cleanup' deletes all built files including .spv files.
• Compiled .spv files now included in release.
• The Vulkan Framework code no longer reads instance extension properties unless it needs to, as this can slow the program.
• Extended header comments added to Median GPU code files.