selftests: gpio: Find libmount with pkg-config if available

If pkg-config is available, use it to define the CFLAGS and
LDLIBS needed for libmount; else, use the current hard-coded
paths and options.

Using pkg-config is very helpful for cross-compilation
environments, and is sometimes readily available on developer
boxes to ensure we get the right compiler/linker options for
the given package.
Signed-off-by: Daniel Díaz <daniel.diaz@linaro.org>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/gpio/Makefile

 # SPDX-License-Identifier: GPL-2.0
 
-CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/
-LDLIBS += -lmount -I/usr/include/libmount
+MOUNT_CFLAGS := $(shell pkg-config --cflags mount 2>/dev/null)
+MOUNT_LDLIBS := $(shell pkg-config --libs mount 2>/dev/null)
+ifeq ($(MOUNT_LDLIBS),)
+MOUNT_LDLIBS := -lmount -I/usr/include/libmount
+endif
+
+CFLAGS += -O2 -g -std=gnu99 -Wall -I../../../../usr/include/ $(MOUNT_CFLAGS)
+LDLIBS += $(MOUNT_LDLIBS)
 
 TEST_PROGS := gpio-mockup.sh
 TEST_FILES := gpio-mockup-sysfs.sh