I don't think the question of "Why not just choose libpari?" ever even presented itself. PARI and MPC both existed, and the latter got donated to the GNU project and so GNU will use it. What's unusual in that?
This is the first release of MPC as a GNU project, so it was originally written independently of the GNU project, so the motives for it being developed in the first place don't come from GNU.
Posted Jul 23, 2012 15:56 UTC (Mon) by JohnLenz (subscriber, #42089)
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I was curious to see how old the projects were, and revision 1 of MPC was January 2003. PARI is a little older at September 1999 but it seems like those early versions of PARI had some wierd non-commercial license. Maybe MPC was started before PARI switched to the GPL? In any case, they are both old projects.
GNU MPC 1.0 "Fagus silvatica" released
Posted Jul 23, 2012 19:27 UTC (Mon) by ballombe (subscriber, #9523)
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PARI/GP first release was in 1986 (before version 1 of the GNU GPL).
The GIT repository start a bit before version 2.0.17 (1999/09/16).
The first release under the GNU GPL was 2.1.0 (2000/11/17).
To cut any speculation: the MPC developers were well-aware of PARI/GP when they decided to write MPC (and conversely, the PARI/GP developers were well-aware of MPC development).