cmCurl: Fix compilation with system curl versions prior to 7.21.5

This version introduced CURLE_NOT_BUILT_IN which we have used since
commit v3.4.0-rc1~211^2~4 (cmCurl: Tolerate lack of CURLOPT_CAPATH
support, 2015-08-12).  For older versions, just define the name to the
then-unused error code so that we can compile.