czmq: fix static build

libstdc++ may also need -lm.

Fixes:
http://autobuild.buildroot.net/results/aab/aab80285c54641fd7c217cdd45c4593e5c3a66c5/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/czmq/czmq.mk

@@ -19,7 +19,7 @@ CZMQ_LICENSE_FILES = LICENSE
 CZMQ_CONF_ENV = ac_cv_prog_czmq_have_asciidoc=no
 
 ifeq ($(BR2_PREFER_STATIC_LIB),y)
-CZMQ_CONF_OPTS += LIBS=-lstdc++
+CZMQ_CONF_OPTS += LIBS="-lstdc++ -lm"
 endif
 
 define CZMQ_CREATE_CONFIG_DIR