This is a breaking update (at least for the internal C++ APIs) so the
packages all need to be updated in lockstep.
Misc notes:
* build2
* Remove unnecessary pkg-config patch
* Trivially refresh remove-config-store-paths.patch
* libbutl
* Enable tests now that they work with NDEBUG
* libodb
* update 2.5.0-b.19 -> 2.5.0-b.21
Build2's dependency libraries and helper programs
bpkg/bdep are all built and installed separately.
Adds a build2 setup hook for packaging purposes,
based on those of cmake and ninja.
Two small patches are applied to build2's source:
one to remove store references from the stored config,
and one to help build2 detect library dirs from NIX_LDFLAGS.
