OpenXR layer's JSON manifest installed in wrong directory on Unix/Linux

[smcv]

On Unix, the JSON manifests describing the Vulkan and OpenXR layers are installed like this:

install(TARGETS VkLayer_gfxreconstruct RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR})
if (UNIX)
    install(FILES $<TARGET_FILE_DIR:VkLayer_gfxreconstruct>/staging-json/VkLayer_gfxreconstruct.json DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/vulkan/explicit_layer.d)
    if (OPENXR_SUPPORT_ENABLED)
        install(FILES $<TARGET_FILE_DIR:VkLayer_gfxreconstruct>/staging-json/XrLayer_gfxreconstruct.json DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/openxr/1/api_layers/explicit_layer.d)
    endif()

In practice this will usually mean /usr/[local/]share/vulkan/explicit_layer.d/ and /usr/[local/]share/openxr/1/api_layers/explicit_layer.d/.

/usr/[local/]share/vulkan/explicit_layer.d/ is correct for Vulkan, the Vulkan loader does look there.

But /usr/[local/]share/openxr/1/api_layers/explicit_layer.d/ is the wrong subdirectory name for OpenXR: according to https://registry.khronos.org/OpenXR/specs/1.1/loader.html#linux-manifest-search-paths it should just be explicit.d. The result is that spec-compliant loaders won't find this layer.

Suggested fix (untested):

-        install(FILES $<TARGET_FILE_DIR:VkLayer_gfxreconstruct>/staging-json/XrLayer_gfxreconstruct.json DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/openxr/1/api_layers/explicit_layer.d)
+        install(FILES $<TARGET_FILE_DIR:VkLayer_gfxreconstruct>/staging-json/XrLayer_gfxreconstruct.json DESTINATION ${CMAKE_INSTALL_DATAROOTDIR}/openxr/1/api_layers/explicit.d)