COMP: Update curl project to fix CMP0054 warning

This commit fixes warnings occurring when using CMake >= 3.1.0

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Author: Jean-Christophe Fillion-Robin <jchris.fillionr@kitware.com>
Date: Wed Jan 21 01:47:56 2015 -0500

Simplify if() conditions on check result variables

Backported from bagder/curl@fba9f41

Remove use of an old hack that takes advantage of the auto-dereference
behavior of the if() command to detect if a variable is defined.  The
hack has the form:

 if("${VAR} MATCHES "^${VAR}$")

where "${VAR}" is a macro argument reference.  Use if(DEFINED) instead.
This also avoids warnings for CMake Policy CMP0054.

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git-svn-id: http://svn.slicer.org/Slicer4/trunk@23894 3bd1e089-480b-0410-8dfb-8563597acbee