Changes

Flag changes and minor fixes for Open XL compile

Some of the remaining compiling issues require some updates
to the existing ZOS flags in order to compile properly using
Open XL for z/OS. Tested to ensure that XLC remains compiling properly
and passes its tests.

Bump to D_OPEN_THREADS=3 as recommended by Open XL devs to enable/expose
pthread functions that may be used and without which can cause compilation
issues.
source: https://www.ibm.com/docs/en/zos/2.5.0?topic=files-feature-test-macros

Signed-off-by: Gaurav Chaudhari <gaurav.chaudhari@ibm.com>
 (commit: 66a405b)