libcurl.m4: AC_PATH_PROG fixes

The invocation of autoconf's AC_PATH_PROG( ) is not quite right for
finding curl-config. This fix corrects the negative case (where
curl-config is not found).

docs/libcurl/libcurl.m4

@@ -75,10 +75,10 @@ AC_DEFUN([LIBCURL_CHECK_CONFIG],
      if test -d "$_libcurl_with" ; then
         LIBCURL_CPPFLAGS="-I$withval/include"
         _libcurl_ldflags="-L$withval/lib"
-        AC_PATH_PROG([_libcurl_config],[curl-config],["$withval/bin"],
+        AC_PATH_PROG([_libcurl_config],[curl-config],[],
                      ["$withval/bin"])
      else
-        AC_PATH_PROG([_libcurl_config],[curl-config])
+        AC_PATH_PROG([_libcurl_config],[curl-config],[],[$PATH])
      fi
 
      if test x$_libcurl_config != "x" ; then