math/mpdecimal: Update 4.0.0 => 4.0.1

Changelog:
https://www.bytereef.org/mpdecimal/changelog.html#version-4-0-1

• Remove GNU_CONFIGURE_MANPREFIX.
• Remove CONFIGURE_ARGS=LD="${CC}" LDXX="${CXX}" - unconditionally

sets LD=$CC and LDXX=$CXX in ./configure now.

• Add USES+=pathfix to fix path for install *.pc files: lib/pkgconfig => libdata/pkgconfig.

PR: 286616
Approved by: Stefan Krah <skrah@bytereef.org> (maintainer)
Co-authored-by: wen <wenheping2000@hotmail.com>