devel/boost: improve port (+)
- Convert ZSTD support to option, default on.
- Support for additional image formats in the Generic Image Library (adds raw, PNG, TIFF, and JPEG formats). Added as an option, default off.
- Resolve the problem preventing our *at() functions (like statat() and openat()) from being detected, it'a Linux-only feature.
- do-test target provided -- it is of limited utility, because it takes multiple days and reports a lot of seemingly false problems.
- Only extract the docs subdirectory, when building boost-docs -- saves some diskspace during builds. Likewise, when building jam (b2) don't extract other code.
- Plist sort, .CURDIR usage optimization.
- Backport a proper fix for broken function program_location_impl in Boost.DLL
PR: 286403, 287204 (based on), 287514 (fix previous version)