build: don't fail on unknown architecture

The autotools build ignored unknown architectures silently while the meson
build errors out. This turns the error into a warning.

Fixes #41
2 jobs for abderrahim/build-fixes in 1 minute and 58 seconds
Status Job ID Name Coverage
  Build
passed #376525
latest-meson-lcms

00:01:58

passed #376526
latest-meson-nolcms

00:01:47