Only use O_CLOEXEC on platforms that support it

Change a9fca58 made use of the O_CLOEXEC flag, which is not supported on
older Linux kernels. This change makes the use contingent on kernel
support.

Testing: I manually compiled breakpad on CentOS 5.8 running kernel
2.6.18-308.8.2.el5.centos.plusxen.

Bug: 730
Change-Id: I21dff928cfba3c156a56708913f65a0c7b5396a6
Reviewed-on: https://chromium-review.googlesource.com/498528
Reviewed-by: Mike Frysinger <vapier@chromium.org>

src/common/linux/guid_creator.cc

@@ -27,6 +27,7 @@
 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+#include "config.h"
 #include "common/linux/eintr_wrapper.h"
 #include "common/linux/guid_creator.h"
 

src/config.h.in

@@ -45,6 +45,9 @@
 /* Define to 1 if you have the <unistd.h> header file. */
 #undef HAVE_UNISTD_H
 
+/* Fallback definition for old systems */
+#undef O_CLOEXEC
+
 /* Name of package */
 #undef PACKAGE