[trac3449] make sure -R has argument (directory) in m4 macro that tests availability.

Bundy issue 6:
(should also be trivial enough, so I'm pushing it directly at my discretion)
without this it's possible that it only results in warning on a system
that doesn't actually support this flag (such as MacOS X) and subsequently
causes build failure.

examples/m4/ax_isc_rpath.m4

@@ -45,8 +45,8 @@ if test x$rpath != xno; then
 	    # Apple clang 5.1 is now considers unknown parameters passed to linker (ld) as errors.
 	    # However, the same unknown parameters passed to compiler (g++ ) are merely threated
 	    # as warnings. To make sure that we pick those up, is to use -Werror.
-            CXXFLAGS="$CXXFLAGS_SAVED -Werror -R"
-            CCFLAGS="$CCFLAGS_SAVED -Werror -R"
+            CXXFLAGS="$CXXFLAGS_SAVED -R/usr/lib"
+            CCFLAGS="$CCFLAGS_SAVED -R/usr/lib"
         AC_TRY_LINK([], [],
             [ AC_MSG_RESULT([yes; note that -R is more sensitive about the position in option arguments])
                 ISC_RPATH_FLAG=-R