Bug#947310: mesa: FTBFS on mipsel: Cannot find (any matches for) "usr/lib/*/libvulkan_*.so"

[Simon McVittie <smcv@debian.org>]

Source: mesa
Version: 19.3.1-2
Severity: serious
Tags: ftbfs
Justification: fails to build from source (but built successfully in the past)

mesa doesn't seem to be building Vulkan drivers on mipsel, but the
Architecture field of mesa-vulkan-drivers says it should be:

https://buildd.debian.org/status/fetch.php?pkg=mesa&arch=mipsel&ver=19.3.1-2&stamp=1576796202&raw=0
>         Vulkan drivers:  no
...
> dh_install -a
> dh_install: Cannot find (any matches for) "usr/share/vulkan/explicit_layer.d/*.json" (tried in ., debian/tmp)
> 
> dh_install: mesa-vulkan-drivers missing files: usr/share/vulkan/explicit_layer.d/*.json
> dh_install: Cannot find (any matches for) "usr/share/vulkan/icd.d/*.json" (tried in ., debian/tmp)
> 
> dh_install: mesa-vulkan-drivers missing files: usr/share/vulkan/icd.d/*.json
> dh_install: Cannot find (any matches for) "usr/lib/*/libvulkan_*.so" (tried in ., debian/tmp)
> 
> dh_install: mesa-vulkan-drivers missing files: usr/lib/*/libvulkan_*.so
> dh_install: Cannot find (any matches for) "usr/lib/*/libVkLayer_MESA_overlay.so" (tried in ., debian/tmp)
> 
> dh_install: mesa-vulkan-drivers missing files: usr/lib/*/libVkLayer_MESA_overlay.so
> dh_install: missing files, aborting

This might be because mipsel is present in the list of architectures where
LLVM is enabled (confflags_GALLIUM += -Dllvm=true etc.), and is present in
the list of architectures where the Vulkan loader is enabled
(confflags_VULKAN += -Dvulkan-overlay-layer=true), but is absent from the
list of architectures where VULKAN_DRIVERS += amd, despite that list being
documented as "only build on the subset of arches where we have LLVM enabled
and where the Vulkan loader is built".

    smcv