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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