osx-attr: add plugin installing config attributes using SystemConfiguration

Currently installs DNS servers only, by prepending IP addresses to the
DNS configuration of the primary networking service.

src/libcharon/Makefile.am

@@ -457,6 +457,13 @@ if MONOLITHIC
 endif
 endif
 
+if USE_OSX_ATTR
+  SUBDIRS += plugins/osx_attr
+if MONOLITHIC
+  libcharon_la_LIBADD += plugins/osx_attr/libstrongswan-osx-attr.la
+endif
+endif
+
 if USE_ANDROID_DNS
   SUBDIRS += plugins/android_dns
 if MONOLITHIC