dev-libs/mpc: revert today's changes for 1.0.3 ebuild, bug #642300

dev-libs/mpc: revert today's changes for 1.0.3 ebuild, bug #642300

This change reverts the following changes:
    commit 0c689de38bcc3b4279aa9e3b09c836e48a237e3d
    dev-libs/mpc: explicitly regenerate ./configure, noticed by Anarchy

    commit cdf7f5c37e8a3dfacfbf5fc81a6a2a8ef3b77a9a
    dev-libs/mpc: fix build failure against mpfr-4.0.0, bug #642300

That way 1.0.3 version will keep failing to build against mpfr-4.0.0
(and prevent users from breaking gcc) and version 1.0.3-r1 will work
as expected.

Reported-by: Attila Tóth
Bug: https://bugs.gentoo.org/642300
Package-Manager: Portage-2.3.19, Repoman-2.3.6