chg: define _GNU_SOURCE to allow CentOS 5 compilation
Without this flag, compilation fails with:
hgclient.c: In function 'hgc_open':
hgclient.c:466: error: 'O_DIRECTORY' undeclared (first use in this function)
hgclient.c:466: error: (Each undeclared identifier is reported only once
hgclient.c:466: error: for each function it appears in.)
Differential Revision: https://phab.mercurial-scm.org/D260
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.