There are, perhaps, other issues as well.

After a build of "master"

a) pip3 install pandas
 - failed to find/download and build numpy
 - after "manual" pip3 install numpy and got to " six, python-dateutil, pytz, pandas"

Then:
 Running setup.py install for pandas ... error
    Complete output from command /opt/bin/python3.8 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-f5rvzoc0/pandas/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-kmi2p9sd/install-record.txt --single-version-externally-managed --compile:
...
    UPDATING build/lib.aix-6.1-3.8-pydebug/pandas/_version.py
    set build/lib.aix-6.1-3.8-pydebug/pandas/_version.py to '0.23.4'
    running build_ext
    building 'pandas._libs.algos' extension
    creating build/temp.aix-6.1-3.8-pydebug
    creating build/temp.aix-6.1-3.8-pydebug/pandas
    creating build/temp.aix-6.1-3.8-pydebug/pandas/_libs
    xlc_r -O -I/opt/include -O2 -qmaxmem=-1 -qarch=pwr5 -I/opt/include -O2 -qmaxmem=-1 -qarch=pwr5 -Ipandas/_libs/src/klib -Ipandas/_libs/src -I/opt/lib/python3.8/site-packages/numpy/core/include -I/opt/include/python3.8dm -c pandas/_libs/algos.c -o build/temp.aix-6.1-3.8-pydebug/pandas/_libs/algos.o -Wno-unused-function
    xlc_r: 1501-210 (S) command option Wno-unused-function contains an incorrect subargument
    error: command 'xlc_r' failed with exit status 40

    ----------------------------------------
Command "/opt/bin/python3.8 -u -c "import setuptools, tokenize;__file__='/tmp/pip-install-f5rvzoc0/pandas/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /tmp/pip-record-kmi2p9sd/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /tmp/pip-install-f5rvzoc0/pandas/

So, it looks like, using xlc - I may not have even gotten as far window.gcc due to issues in pandas (the incorrect flag -Wno-used-function - have to find where that came from!)