fix macro HAVE_O_CLOEXEC when O_CLOEXEC not found

[emptyx0]

port_config.h define HAVE_O_CLOEXEC is 0 when O_CLOEXEC not found in fcntl.h, so condition '#if defined(HAVE_O_CLOEXEC)' hit. It could be verified on centos5

[google-cla[bot]]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[emptyx0]

macro HAVE_O_CLOEXEC is defined automatic by cmake in file port_config.h.in, cmake will define HAVE_O_CLOEXEC equals 0, so #if directive on HAVE_O_CLOEXEC in file env_posix.cc should be '#if HAVE_O_CLOEXEC' , not '#if defined(HAVE_O_CLOEXEC)',

[maflcko]

@emptyx0 Did you want to sign the CLA?

[emptyx0]

@emptyx0 Did you want to sign the CLA?

I have already signed the CLA , 3Q dalao

[hebasto]

Also see https://github.com/google/leveldb/pull/1149.