selftests: alsa: Handle pkg-config failure more gracefully

Follow the pattern used by other selftests like memfd and fall back on the
standard toolchain options to build with a system installed alsa-lib if
we don't get anything from pkg-config. This reduces our build dependencies
a bit in the common case while still allowing use of pkg-config in case
there is a need for it.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220531151337.2933810-1-broonie@kernel.org


Signed-off-by: Takashi Iwai <tiwai@suse.de>