toolchain-external: improve musl external check

The current test to verify if the toolchain uses musl or not is based on
checking if /lib/libc.so or /lib/libm.so exist in the sysroot. However,
some toolchains (notably Crosstool-NG ones) put these libraries in
/usr/lib/.

To fix this, build a minimal C program and check if the program
interpreter contains /lib/ld-musl.

Signed-off-by: Ilya Kuzmich <ilya.kuzmich@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

toolchain/helpers.mk

@@ -197,13 +197,18 @@ check_glibc = \
 #
 # Check that the selected C library really is musl
 #
-# $1: sysroot directory
+# $1: cross-gcc path
+# $2: cross-readelf path
 check_musl = \
-	SYSROOT_DIR="$(strip $1)"; \
-	if test ! -f $${SYSROOT_DIR}/lib/libc.so -o -e $${SYSROOT_DIR}/lib/libm.so ; then \
+	__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*; \
 		echo "Incorrect selection of the C library" ; \
 		exit -1; \
-	fi
+	fi ; \
+	rm -f $(BUILD_DIR)/.br-toolchain-test.tmp*
 
 #
 # Check the conformity of Buildroot configuration with regard to the

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

@@ -565,7 +565,9 @@ define $(2)_CONFIGURE_CMDS
 	if test "$$(BR2_TOOLCHAIN_EXTERNAL_UCLIBC)" = "y" ; then \
 		$$(call check_uclibc,$$$${SYSROOT_DIR}) ; \
 	elif test "$$(BR2_TOOLCHAIN_EXTERNAL_MUSL)" = "y" ; then \
-		$$(call check_musl,$$$${SYSROOT_DIR}) ; \
+		$$(call check_musl,\
+			"$$(TOOLCHAIN_EXTERNAL_CC) $$(TOOLCHAIN_EXTERNAL_CFLAGS)",\
+			$$(TOOLCHAIN_EXTERNAL_READELF)) ; \
 	else \
 		$$(call check_glibc,$$$${SYSROOT_DIR}) ; \
 	fi