Found CMake: /usr/bin/cmake (3.22.4)
WARNING: CMake Toolchain: Failed to determine CMake compilers state
Run-time dependency unity found: NO (tried pkgconfig and cmake)
Run-time dependency gdk-x11-3.0 found: YES 3.24.34
Run-time dependency gdk-wayland-3.0 found: NO (tried pkgconfig and cmake)

meson.build:67:0: ERROR: Dependency "gdk-wayland-3.0" not found, tried pkgconfig and cmake

A full log can be found at /var/tmp/portage/media-gfx/shotwell-0.30.15/work/shotwell-0.30.15-build/meson-logs/meson-log.txt
 * ERROR: media-gfx/shotwell-0.30.15::gentoo failed (configure phase):
 *   (no error message)
 * 
 * Call stack:
 *     ebuild.sh, line  127:  Called src_configure
 *   environment, line 2720:  Called meson_src_configure
 *   environment, line 2049:  Called die
 * The specific snippet of code:
 *       "${mesonargs[@]}" ) || die
 * 
 * If you need support, post the output of `emerge --info '=media-gfx/shotwell-0.30.15::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=media-gfx/shotwell-0.30.15::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/media-gfx/shotwell-0.30.15/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/media-gfx/shotwell-0.30.15/temp/environment'.
 * Working directory: '/var/tmp/portage/media-gfx/shotwell-0.30.15/work/shotwell-0.30.15'
 * S: '/var/tmp/portage/media-gfx/shotwell-0.30.15/work/shotwell-0.30.15'

>>> Failed to emerge media-gfx/shotwell-0.30.15, Log file:

>>>  '/var/tmp/portage/media-gfx/shotwell-0.30.15/temp/build.log'

 * Messages for package media-gfx/shotwell-0.30.15:

 * ERROR: media-gfx/shotwell-0.30.15::gentoo failed (configure phase):
 *   (no error message)
 * 
 * Call stack:
 *     ebuild.sh, line  127:  Called src_configure
 *   environment, line 2720:  Called meson_src_configure
 *   environment, line 2049:  Called die
 * The specific snippet of code:
 *       "${mesonargs[@]}" ) || die
 * 
 * If you need support, post the output of `emerge --info '=media-gfx/shotwell-0.30.15::gentoo'`,
 * the complete build log and the output of `emerge -pqv '=media-gfx/shotwell-0.30.15::gentoo'`.
 * The complete build log is located at '/var/tmp/portage/media-gfx/shotwell-0.30.15/temp/build.log'.
 * The ebuild environment file is located at '/var/tmp/portage/media-gfx/shotwell-0.30.15/temp/environment'.
 * Working directory: '/var/tmp/portage/media-gfx/shotwell-0.30.15/work/shotwell-0.30.15'
 * S: '/var/tmp/portage/media-gfx/shotwell-0.30.15/work/shotwell-0.30.15'

https://bugs.gentoo.org/836831

Ein Eintrag in /etc/portage/package.use/shotwell mit folgenden USE-Flags bringen die Lösung

# shotwell
media-libs/gst-plugins-bad         egl
x11-libs/gtk+                      wayland
media-libs/mesa                    wayland

Danach mit emerge -avuND ein Update durchführen