Build: resolve build error with vulkan_loader not finding Wayland

build_files/build_environment/cmake/vulkan.cmake

@@ -30,6 +30,17 @@ set(VULKAN_LOADER_EXTRA_ARGS
   -DVULKAN_HEADERS_INSTALL_DIR=${LIBDIR}/vulkan_headers
 )
 
+if(UNIX AND NOT APPLE)
+  # These are used in `cmake/FindWayland.cmake` from `external_vulkan_loader`.
+  # NOTE: When upgrading to CMAKE 3.22 we it would be cleaner to use: `PKG_CONFIG_ARGN`,
+  # so `pkgconfig` would find wayland.
+  set(VULKAN_LOADER_EXTRA_ARGS
+    ${VULKAN_LOADER_EXTRA_ARGS}
+    -DPKG_WAYLAND_INCLUDE_DIRS=${LIBDIR}/wayland/include
+    -DPKG_WAYLAND_LIBRARY_DIRS=${LIBDIR}/wayland/lib64
+  )
+endif()
+
 ExternalProject_Add(external_vulkan_loader
   URL file://${PACKAGE_DIR}/${VULKAN_LOADER_FILE}
   URL_HASH ${VULKAN_LOADER_HASH_TYPE}=${VULKAN_LOADER_HASH}
@@ -43,7 +54,12 @@ add_dependencies(
   external_vulkan_headers
 )
 
-if(WIN32)
+if(UNIX AND NOT APPLE)
+  add_dependencies(
+    external_vulkan_loader
+    external_wayland
+  )
+elseif(WIN32)
   if(BUILD_MODE STREQUAL Release)
     ExternalProject_Add_Step(external_vulkan_loader after_install
       COMMAND ${CMAKE_COMMAND} -E copy_directory ${LIBDIR}/vulkan_loader/ ${HARVEST_TARGET}/vulkan