selftest: alsa: check if user has alsa installed

Currently, if alsa development package is not installed on the user's
system then the make command would print a `pagefull` of errors. In
particular one error message is repeated 3 times. This error is returned
by `pkg-config` and since it is not being handeled appropriately,
repeated calls to `pkg-config` prints the same message again.

This patch adds check for alsa package installation. If alsa is not
installed, a short and consize error is returned. Also, it does not
affect the compilation of other tests.

Signed-off-by: Abdul Rahim <[e-mail]>
Reviewed-by: Shuah Khan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

tools/testing/selftests/alsa/Makefile

···
 # SPDX-License-Identifier: GPL-2.0
 #
+ifneq ($(shell pkg-config --exists alsa && echo 0 || echo 1),0)
+$(error Package alsa not found, please install alsa development package or \
+	add directory containing `alsa.pc` in PKG_CONFIG_PATH)
+endif
 
 CFLAGS += $(shell pkg-config --cflags alsa) $(KHDR_INCLUDES)
 LDLIBS += $(shell pkg-config --libs alsa)