libcurl.m4: include the standard source header

#***************************************************************************

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#  Project                     ___| | | |  _ \| |

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#

# Copyright (C) 2006, David Shaw <dshaw@jabberwocky.com>

#

# This software is licensed as described in the file COPYING, which

# you should have received as part of this distribution. The terms

# are also available at http://curl.haxx.se/docs/copyright.html.

#

# You may opt to use, copy, modify, merge, publish, distribute and/or sell

# copies of the Software, and permit persons to whom the Software is

# furnished to do so, under the terms of the COPYING file.

#

# This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY

# KIND, either express or implied.

#

###########################################################################

# LIBCURL_CHECK_CONFIG ([DEFAULT-ACTION], [MINIMUM-VERSION],

#                       [ACTION-IF-YES], [ACTION-IF-NO])

# ----------------------------------------------------------