build: pad header for osx libs

This ensures that users of the lib will be able to mangle the paths to work
in their bundles.

configure.ac

@@ -285,6 +285,7 @@ case $host in
        esac
      fi
 
+     AX_CHECK_LINK_FLAG([[-Wl,-headerpad_max_install_names]], [LDFLAGS="$LDFLAGS -Wl,-headerpad_max_install_names"])
      CPPFLAGS="$CPPFLAGS -DMAC_OSX"
      ;;
    *linux*)