msgpack: fix static build

Disabled shared libraries when building static only.

Fixes:
http://autobuild.buildroot.net/results/057/057d72bc84c467973ec3c076f5d8108e02d6e283/
http://autobuild.buildroot.net/results/499/499069f6821ddd2dcb83201b017a58eae00ebe16/
http://autobuild.buildroot.net/results/499/499069f6821ddd2dcb83201b017a58eae00ebe16/

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

package/msgpack/msgpack.mk

@@ -11,4 +11,8 @@ MSGPACK_LICENSE_FILES = COPYING LICENSE_1_0.txt
 MSGPACK_INSTALL_STAGING = YES
 MSGPACK_CONF_OPTS = -DMSGPACK_BUILD_EXAMPLES=OFF
 
+ifeq ($(BR2_STATIC_LIBS),y)
+MSGPACK_CONF_OPTS += -DMSGPACK_ENABLE_SHARED=OFF
+endif
+
 $(eval $(cmake-package))