Merge branch 'rs/use-enhanced-bre-on-macos'

Newer regex library macOS stopped enabling GNU-like enhanced BRE,
where '\(A\|B\)' works as alternation, unless explicitly asked with
the REG_ENHANCED flag.  "git grep" now can be compiled to do so, to
retain the old behaviour.

* rs/use-enhanced-bre-on-macos:
  use enhanced basic regular expressions on macOS

diff --cc Makefile
Simple merge

diff --cc git-compat-util.h
Simple merge