m4/gpg4win: Add /usr/host/bin folder in library search list.

The /usr/$host/bin folder is used in ArchLinux. See the file list
of mingw-w64-gcc package here:
https://www.archlinux.org/packages/community/x86_64/mingw-w64-gcc/files/

• Signed-off-by: Biswapriyo Nath <nathbappai@gmail.com>