_GNU_SOURCE should be defined for building vs uClibc

_GNU_SOURCE should be defined for building vs uClibc

In uClibc-ng O_PATH and O_DIRECTORY are only defined if _GNU_SOURCE is
defined.

So explicitly define _GNU_SOURCE in sources that use O_PATH and
O_DIRECTORY.

Without that extra definition that's what happens when building procd.

utils/utils.c:
------------------------->8----------------------
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/utils/utils.c:
In function 'patch_fd':
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/utils/utils.c:168:22:
error: 'O_PATH' undeclared (first use in this function)
   dfd = open("/dev", O_PATH|O_DIRECTORY);
                      ^
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/utils/utils.c:168:22:
note: each undeclared identifier is reported only once for each function
it appears in
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/utils/utils.c:168:29:
error: 'O_DIRECTORY' undeclared (first use in this function)
   dfd = open("/dev", O_PATH|O_DIRECTORY);
                             ^
CMakeFiles/init.dir/build.make:182: recipe for target
'CMakeFiles/init.dir/utils/utils.c.o' failed
------------------------->8----------------------

inittab.c:
------------------------->8----------------------
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/inittab.c:
In function 'dev_exist':
.../openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/inittab.c:72:21:
error: 'O_PATH' undeclared (first use in this function)
  dfd = open("/dev", O_PATH|O_DIRECTORY);
                     ^
.../git/openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/inittab.c:72:21:
note: each undeclared identifier is reported only once for each function
it appears in
.../git/openwrt/build_dir/target-arc_arc700_uClibc-1.0.14/procd-2016-05-19/inittab.c:72:28:
error: 'O_DIRECTORY' undeclared (first use in this function)
  dfd = open("/dev", O_PATH|O_DIRECTORY);
                            ^
CMakeFiles/procd.dir/build.make:134: recipe for target
'CMakeFiles/procd.dir/inittab.c.o' failed
make[6]: *** [CMakeFiles/procd.dir/inittab.c.o] Error 1
------------------------->8----------------------

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Waldemar Brodkorb <wbx@openadk.org>
Cc: John Crispin <john@phrozen.org>
Cc: Jo-Philipp Wich <jo@mein.io>