Add option to prepend libdir to library's path in vulkan manifest

"\$LIB" technically breaks json but on the other hand vulkan loader
currently doesn't handle "\\$LIB"
Help with issue #468

meson_options.txt

@@ -3,6 +3,7 @@ option('use_system_vulkan', type : 'feature', value : 'disabled', description: '
 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')
+option('prepend_libdir_vk', type : 'boolean', value : true, description: 'Prepend libdir to library path in vulkan manifest')
 option('include_doc', type : 'boolean', value : true, description: 'Include the example config')
 option('with_nvml', type : 'combo', value : 'enabled', choices: ['enabled', 'system', 'disabled'], description: 'Enable NVML support')
 option('with_xnvctrl', type : 'feature', value : 'enabled', description: 'Enable XNVCtrl support')

src/meson.build

@@ -25,6 +25,11 @@ else
   conf_data.set('ld_libdir_mangohud', ld_libdir_mangohud)
 endif
 
+ld_libdir_mangohud_vk = ''
+if get_option('prepend_libdir_vk')
+  ld_libdir_mangohud_vk = ld_libdir_mangohud
+endif
+
 overlay_shaders = [
   'overlay.frag',
   'overlay.vert',
@@ -207,7 +212,7 @@ endif
 
 configure_file(input : 'mangohud.json.in',
   output : '@0@.json'.format(meson.project_name()),
-  configuration : {'ld_libdir_mangohud' : ld_libdir_mangohud,
+  configuration : {'ld_libdir_mangohud' : ld_libdir_mangohud_vk,
                   'PROJECT_NAME' : meson.project_name().to_upper()},
   install : true,
   install_dir : join_paths(get_option('datadir'), 'vulkan', 'implicit_layer.d'),