cjson: link with libm, unavailable for static

Link with libm as stated in the README, fixes:
http://autobuild.buildroot.net/results/57f/57fc124e14263ee2447e20a5b910ed3ae0a5b7db/

Also disable the package for static builds since it doesn't work that
way.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/cjson/Config.in

@@ -1,5 +1,7 @@
 config BR2_PACKAGE_CJSON
 	bool "cJSON"
+	# For static build with cJSON.c directly see README
+	depends on !BR2_PREFER_STATIC_LIB
 	help
 	  An ultra-lightweight, portable, single-file, simple-as-can-be ANSI-C
 	  compliant JSON parser, under MIT license.

package/cjson/cjson.mk

@@ -12,7 +12,8 @@ CJSON_LICENSE         = MIT
 CJSON_LICENSE_FILES   = LICENSE
 
 define CJSON_BUILD_CMDS
-	cd $(@D) && $(TARGET_CC) $(TARGET_CFLAGS) -shared -fPIC cJSON.c -o libcJSON.so
+	cd $(@D) && $(TARGET_CC) $(TARGET_CFLAGS) -shared -fPIC -lm \
+		cJSON.c -o libcJSON.so
 endef
 
 define CJSON_INSTALL_STAGING_CMDS