mpdecimal: fix build on pure static library configurations

This commit adds a patch to mpdecimal that makes it understand
--enable-shared/--disable-shared and --enable-static/--disable-static,
even if mpdecimal isn't using automake/libtool. It allows to build
only the static variant of the mpdecimal library when needed.

Fixes:

  http://autobuild.buildroot.org/results/276/2764bd8c42ba659682760ffc6afa933b7530d06c/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>