Fix bug in _gcry_mpi_tdiv_q_2exp.

* mpi/mpi-internal.h (MPN_COPY_INCR): Make it work.
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This bug has been with us since the version 0.0.0 of GnuPG.
Fortunately it only affects an optimized code path which is rarely
used in practice: If the shift size matches the size of a
limb (i.e.. 32 or 64); this is is_prime in primegen.c.  Over there the
Rabin-Miller test may fail with a probability of 2^-31 (that is if the
to be tested prime - 1 has the low 32 bits cleared).  In practice the
probability is even much less because we first do a Fermat test on the
randomly generated candidates which sorts out the majority of
composite numbers.

The bug in MPN_COPY_INCR was found by Sven Bjorn.

Signed-off-by: Werner Koch <wk@gnupg.org>

(cherry picked from commit a7a9cdcaaf3979baa18dad51e722882581349f45)
w32: Fix installing of .def file.

* src/Makefile.am (install-def-file): Create libdir first.
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Reported-by: LRN <lrn1986@gmail.com>
(cherry picked from commit 4e46d8bc78008ba06f106b368cefb0dddf15fe38)