import mpdecimal-2.4.0 for DFP math

    Faster integer to string conversion.
    mpd_qln(), mpd_qlog10() and mpd_pow() are now thread-safe.
    All functions that take or return C integers are now available in both the 64-bit and the 32-bit builds.
    Support for cross-compiling.
    Scripts for Visual Studio builds.
Prepare for mingw-w64 build integration.

Changes:
* Make memory allocator callback const, use HeapAlloc and friends.
* Mangle all external symbols with __mingw_dfp_ prefix.
* include mpdecimal-i686.h or mpdecimal-x86_64.h depending on platform.
Point to new mpdecimal

Remove old 2.3 mpdecimal build.
Regenerate automake files
Remove obsolete mpdecimal-2.3