meson: Add vulkan_datadir option (#522)

This option enables using system Vulkan headers when it doesn't share
the same datadir as MangoHud. This is specifically an issue in NixOS
where packages are built into their own isolated directories in
/nix/store.

For example:
/nix/store/9g28nz6zh484144mzw670bx82y1ka49a-vulkan-headers-1.2.162.0/share/vulkan/registry/vk.xml

This is currently worked around in the Nix package, by symlinking the
vulkan-headers into MangoHud's install directory:

5debc57760/pkgs/tools/graphics/mangohud/default.nix (L66-L70)

Although, this hack causes the vulkan-headers to be included as a
runtime dependency when they're only needed at build time.

I tried to figure out a way to resolve the datadir automatically, but
found that just adding a new option is probably the best solution for
the following reasons:

- The vulkan.pc file used to resolve dep_vulkan doesn't contain any
information about the datadir.

- The share directory could be resolved relative to the vulkan include
directory and that would solve the specific issue for Nix, but the
datadir could be installed anywhere and that wouldn't be a general
solution.

- I couldn't find a way to resolve the data dir using the XDG Base
Directory Specification because meson doesn't provide a way to access
the necessary environment variables.

https://specifications.freedesktop.org/basedir-spec/basedir-spec-latest.html

meson.build

@@ -183,7 +183,10 @@ else
 endif
 
 if dep_vulkan.found()
-  datadir = get_option('datadir')
+  datadir = get_option('vulkan_datadir')
+  if datadir == ''
+    datadir = get_option('datadir')
+  endif
   if not datadir.startswith('/')
     datadir = get_option('prefix') / datadir
   endif

meson_options.txt

@@ -1,5 +1,6 @@
 option('glibcxx_asserts', type : 'boolean', value : false)
 option('use_system_vulkan', type : 'feature', value : 'disabled', description: 'Use system vulkan headers instead of the provided ones')
+option('vulkan_datadir', type : 'string', value : '', description: 'Path to the system vulkan headers data directory if different from MangoHud\'s datadir')
 option('append_libdir_mangohud', type : 'boolean', value : true, description: 'Append "mangohud" to libdir path or not.')
 option('ld_libdir_prefix', type : 'boolean', value : false, description: 'Set ld libdir to "$prefix/lib/mangohud/\$LIB"')
 option('ld_libdir_abs', type : 'boolean', value : false, description: 'Use absolute path in LD_PRELOAD')