toolchain: improve musl check to support static toolchains

The check_musl function currently builds a program and verifies if the
program interpreter starts with /lib/ld-musl. While this works fine
for dynamically linked programs, this obviously doesn't work for a
purely static musl toolchain such as [1].

There is no easy way to identify a toolchain as using the musl C
library. For glibc, dynamic linking is always supported, so we look at
the dynamic linker name. For uClibc, there is a distinctive
uClibc_config.h header file. There is no such distinctive feature in
musl.

We end up resorting to looking for the string MUSL_LOCPATH, which is
used by musl locale_map.c source file. This string has been present in
musl since 2014. It certainly isn't a very stable or convincing
solution to identify the C library as being musl, but it's the best we
could find.

Note that we are sure there is a libc.a file, because the
check_unusable_toolchain function checks that there is a such a file.

[1] http://autobuild.buildroot.net/toolchains/tarballs/br-arm-musl-static-2018.05.tar.bz2

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

toolchain/helpers.mk

@@ -241,14 +241,11 @@ check_glibc = \
 # $2: cross-readelf path
 check_musl = \
 	__CROSS_CC=$(strip $1) ; \
-	__CROSS_READELF=$(strip $2) ; \
-	echo 'void main(void) {}' | $${__CROSS_CC} -x c -o $(BUILD_DIR)/.br-toolchain-test.tmp - >/dev/null 2>&1; \
-	if ! $${__CROSS_READELF} -l $(BUILD_DIR)/.br-toolchain-test.tmp 2> /dev/null | grep 'program interpreter: /lib/ld-musl' -q; then \
-		rm -f $(BUILD_DIR)/.br-toolchain-test.tmp*; \
+	libc_a_path=`$${__CROSS_CC} -print-file-name=libc.a` ; \
+	if ! strings $${libc_a_path} | grep -q MUSL_LOCPATH ; then \
 		echo "Incorrect selection of the C library" ; \
 		exit -1; \
-	fi ; \
-	rm -f $(BUILD_DIR)/.br-toolchain-test.tmp*
+	fi
 
 #
 # Check the conformity of Buildroot configuration with regard to the

toolchain/toolchain-external/pkg-toolchain-external.mk

@@ -557,8 +557,7 @@ define $(2)_CONFIGURE_CMDS
 		$$(call check_uclibc,$$$${SYSROOT_DIR}) ; \
 	elif test "$$(BR2_TOOLCHAIN_EXTERNAL_MUSL)" = "y" ; then \
 		$$(call check_musl,\
-			"$$(TOOLCHAIN_EXTERNAL_CC) $$(TOOLCHAIN_EXTERNAL_CFLAGS)",\
-			$$(TOOLCHAIN_EXTERNAL_READELF)) ; \
+			"$$(TOOLCHAIN_EXTERNAL_CC) $$(TOOLCHAIN_EXTERNAL_CFLAGS)") ; \
 	else \
 		$$(call check_glibc,$$$${SYSROOT_DIR}) ; \
 	fi