package/libmemcached: link with -latomic when needed

Fix the following build failure raised since bump to version 1.1.4 in
commit 7205df8a4f3c729b11a5f0c34885e6cf592f24b9:

/home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/11.3.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: CMakeFiles/aslap.dir/ms_conn.c.o: undefined reference to symbol '__atomic_fetch_add_4@@LIBATOMIC_1.0'

Fixes:
 - http://autobuild.buildroot.org/results/c8e4e1f9609d1339fe070afe440c63660892600e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libmemcached/libmemcached.mk

@@ -32,4 +32,8 @@ else
 LIBMEMCACHED_CONF_OPTS += -DENABLE_OPENSSL_CRYPTO=OFF
 endif
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+LIBMEMCACHED_CONF_OPTS += -DCMAKE_EXE_LINKER_FLAGS=-latomic
+endif
+
 $(eval $(cmake-package))