cc-v1.6.0

Added

• add Build::create_archive and emit_link_directives (#1972)
• read the process environment once per Build, on first use (#1969)
• add Build::message_logger for structured access to cc's messages (#1967)
• add CXXSTDLIB_STATIC to link the C++ stdlib statically from outside (#1957)

Fixed

• key the flag support cache on target, host, language and environment (#1965)
• forward each line of compiler stderr once and whole with parallel (#1959)

Other

• find compile commands by source file so tests pass with parallel (#1973)
• add support for armeb-unknown-linux-gnueabi (#1958)