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

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