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

contrib/chg/Makefile

@@ -5,7 +5,7 b' SRCS = chg.c hgclient.c procutil.c util.'
 OBJS = $(SRCS:.c=.o)
 
 CFLAGS ?= -O2 -Wall -Wextra -pedantic -g
-CPPFLAGS ?= -D_FORTIFY_SOURCE=2
+CPPFLAGS ?= -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE
 override CFLAGS += -std=gnu99
 ifdef HGPATH
 override CPPFLAGS += -DHGPATH=\"$(HGPATH)\"