protobuf: link with libatomic when needed

The protobuf library uses atomic intrinsics, so we need to link
against libatomic.

Fixes the build of protobuf on Sparc:

  http://autobuild.buildroot.net/results/f3d76eaebd529a61bce849e355182c60f233ed06/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/protobuf/protobuf.mk

@@ -17,6 +17,10 @@ PROTOBUF_LICENSE_FILES = LICENSE
 PROTOBUF_DEPENDENCIES = host-protobuf
 PROTOBUF_CONF_OPTS = --with-protoc=$(HOST_DIR)/usr/bin/protoc
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+PROTOBUF_CONF_ENV += LIBS=-latomic
+endif
+
 PROTOBUF_INSTALL_STAGING = YES
 
 PROTOBUF_PATCH = https://github.com/google/protobuf/commit/416f90939d4de58fe1a4e2489120010313183291.patch