coreutils: disable stdbuf for static builds

When stdbuf is enabled, it builds a shared library, but it will fail
in a static-only scenario.

Fixes:

  http://autobuild.buildroot.org/results/974/9741cc9841fa95f7430a91f585c4464ee2c9311b/

Signed-off-by: David du Colombier <0intro@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/coreutils/coreutils.mk

@@ -109,6 +109,10 @@ endef
 COREUTILS_POST_INSTALL_TARGET_HOOKS += COREUTILS_CLEANUP_BIN
 endif
 
+ifeq ($(BR2_STATIC_LIBS),y)
+COREUTILS_CONF_OPTS += --enable-no-install-program=stdbuf
+endif
+
 define COREUTILS_CLEANUP
 	# link for archaic shells
 	ln -fs test $(TARGET_DIR)/usr/bin/[