sane-backends: unbreak on toolchains without threads support

Fixes:
http://autobuild.buildroot.net/results/f2a/f2a43b62aa2a720c538fc60c80c78c8ed5b04dea/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/sane-backends/sane-backends.mk

@@ -11,6 +11,9 @@ SANE_BACKENDS_LICENSE = GPLv2+
 SANE_BACKENDS_LICENSE_FILES = COPYING
 SANE_BACKENDS_INSTALL_STAGING = YES
 
+SANE_BACKENDS_CONF_OPTS = \
+	$(if $(BR2_TOOLCHAIN_HAS_THREADS),--enable-pthread,--disable-pthread)
+
 ifeq ($(BR2_PACKAGE_LIBUSB),y)
 SANE_BACKENDS_DEPENDENCIES += libusb
 SANE_BACKENDS_CONF_OPTS += --enable-libusb_1_0