Update depends to use an absolute path.

Summary: This is required for out of tree builds. When not used, one may end up with path starting in ../depends and things start to fail all over the place.

Test Plan:
From the root folder:

make HOST=i686-w64-mingw32
CONFIG_SITE=../depends/i686-w64-mingw32/share/config.site ../configure
make

Reviewers: schancel, freetrader, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D770