Fix mpfr_custom_get_kind() macro bug.

* src/mpfr.h: in the mpfr_custom_get_kind() macro, changed mpfr_ptr to
  mpfr_srcptr for _x to agree with the function prototype, in order to
  avoid a compilation failure of user code in some cases. This bug was
  introduced by commit 9f94e031, which
  introduced the temporary variable _x to avoid an incorrect number of
  evaluations of the x argument.
* tests/tstckintc.c: improved the tests to detect this bug.

This should fix mpfr bug #1.

Bug initially reported by FX Coudert:
  https://github.com/CGAL/cgal/issues/7064

It affects Fedora Linux:
  https://bugzilla.redhat.com/show_bug.cgi?id=2144197