Merge branch 'ps/build' into ps/ci-meson

* ps/build: (24 commits) Introduce support for the Meson build system Documentation: add comparison of build systems t: allow overriding build dir t: better support for out-of-tree builds Documentation: extract script to generate a list of mergetools Documentation: teach "cmd-list.perl" about out-of-tree builds Documentation: allow sourcing generated includes from separate dir Makefile: simplify building of templates Makefile: write absolute program path into bin-wrappers Makefile: allow "bin-wrappers/" directory to exist Makefile: refactor generators to be PWD-independent Makefile: extract script to generate gitweb.js Makefile: extract script to generate gitweb.cgi Makefile: extract script to massage Python scripts Makefile: extract script to massage Shell scripts Makefile: use "generate-perl.sh" to massage Perl library Makefile: extract script to massage Perl scripts Makefile: consistently use PERL_PATH Makefile: generate doc versions via GIT-VERSION-GEN Makefile: generate "git.rc" via GIT-VERSION-GEN ...