gcc's implementation of the C++17 file system library doesn't support
UNC paths of style `\\?\…` on Windows. There doesn't seem to be any
progress towards implementing support for it.
Fixes#3058. See also #2916.
Add build support for a minimal boost tree which contains only
filesystem, regex and system dynamic libraries plus all headers.
- merge latest autoconf macros for boost from autoconf archive
- patch { filesystem, regex, system } macros to detect .dylib suffixes
- platform: Mac OS X 10.7.2
- boost versions: 1.47.0, 1.48.0
- compiler: gcc-4.6.2
