pythongh-115119: Recommend upstream libmpdec in build requirements (python#136205)

Co-authored-by: Stan Ulbrych <[email protected]>
Co-authored-by: Victor Stinner <[email protected]>
(cherry picked from commit b2e498a)

Author: 3 people

Doc/using/configure.rst

@@ -32,6 +32,9 @@ Features and minimum versions required to build CPython:
 * Autoconf 2.71 and aclocal 1.16.5 are required to regenerate the
   :file:`configure` script.
 
+* `libmpdec <https://www.bytereef.org/mpdecimal/doc/libmpdec/>`_ 2.5.0
+  for the :mod:`decimal` module.
+
 .. versionchanged:: 3.1
    Tcl/Tk version 8.3.1 is now required.