contrib: Add support for out-of-tree builds in gen-manpages.sh

Summary:
This adds support for setting the environment variable BUILDDIR
to point to executables that are outside the source directory.

E.g. to invoke the tool when the build is in $PWD/build:

bash
BUILDDIR=$PWD/build contrib/devtools/gen-manpages.sh

Test Plan:

BUILDDIR=$PWD/build-cmake contrib/devtools/gen-manpages.sh

New manpages should be generated.

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2271