component/libcap: support the case where lib64 directory doesn't exist

the previous version was failing like this if lib64 doesn't exist:

libcap: Command 'set -e;cd /opt/slapgrid/shared/libcap/4d09237abd7cb34117fccc1d3c3bc862
[ -d lib64 ] && ln -s lib64 lib' returned non-zero exit status 1

component/libcap/buildout.cfg

@@ -23,10 +23,10 @@ make-options =
 environment =
   PATH=${gperf:location}/bin:%(PATH)s
 
-# if lib64 exists, then create a symlink from lib to lib64
+# if lib doesn't exist, then create a symlink 'lib' pointing to lib64
 post-install =
   cd ${:location}
-  [ -d lib64 ] && ln -s lib64 lib
+  [ -d lib ] || ln -s lib64 lib
 
 [libcap]
 <= libcap2