samples: auxdisplay: use 'userprogs' syntax

Kbuild now supports the 'userprogs' syntax to compile userspace
programs for the same architecture as the kernel.

Add the entry to samples/Makefile to put this into the build bot
coverage.

I also added the CONFIG option guarded by 'depends on CC_CAN_LINK'
because $(CC) may not provide libc.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Acked-by: Sam Ravnborg <sam@ravnborg.org>

samples/Kconfig

@@ -6,6 +6,10 @@ menuconfig SAMPLES
 
 if SAMPLES
 
+config SAMPLE_AUXDISPLAY
+	bool "auxdisplay sample"
+	depends on CC_CAN_LINK
+
 config SAMPLE_TRACE_EVENTS
 	tristate "Build trace_events examples -- loadable modules only"
 	depends on EVENT_TRACING && m

samples/Makefile

@@ -1,6 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0
 # Makefile for Linux samples code
 
+subdir-$(CONFIG_SAMPLE_AUXDISPLAY)	+= auxdisplay
 obj-$(CONFIG_SAMPLE_ANDROID_BINDERFS)	+= binderfs/
 obj-$(CONFIG_SAMPLE_CONFIGFS)		+= configfs/
 obj-$(CONFIG_SAMPLE_CONNECTOR)		+= connector/

samples/auxdisplay/Makefile

@@ -1,10 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
-CC := $(CROSS_COMPILE)gcc
-CFLAGS := -I../../usr/include
-
-PROGS := cfag12864b-example
-
-all: $(PROGS)
-
-clean:
-	rm -fr $(PROGS)
+userprogs := cfag12864b-example
+always-y := $(userprogs)