Daniel Johnson's fix for shared object extension detection on Mac OS X.

acinclude.m4

@@ -1861,6 +1861,7 @@ AC_DEFUN([CURL_DLLIB_NAME],
 AC_MSG_CHECKING([name of dynamic library $2])
 dnl The shared library extension variable name changes from version to
 dnl version of libtool.  Try a few names then just set one statically.
+test -z "$shared_ext" && eval shared_ext=\"$shrext_cmds\"
 test -z "$shared_ext" && shared_ext="$std_shrext"
 test -z "$shared_ext" && shared_ext="$shrext"
 test -z "$shared_ext" && shared_ext=".so"