No fastcall attribute in POWER platform
Compiling the development branch of pandas fail in AIX. 

pandas/_libs/src/ujson/lib/ultrajsonenc.c:397:1: error: ‘fastcall’ attribute directive ignored [-Werror=attributes]
 Buffer_AppendShortHexUnchecked(char *outputOffset, unsigned short value) {
 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
pandas/_libs/src/ujson/lib/ultrajsonenc.c:726:59: error: ‘fastcall’ attribute directive ignored [-Werror=attributes]
                                                           char *end) {
                                                           ^~~~
cc1: all warnings being treated as errors


The fastcall attribute is not available in POWER platform also. 


FWIW I would be OK with removing this altogether. AFAIK it's only for i386 platforms which we don't really care to optimize at this point > FWIW I would be OK with removing this altogether. AFAIK it's only for i386 platforms which we don't really care to optimize at this point

Okay, so shall i remove it altogether in this PR ? Yea let's go ahead and remove it. If you can run the JSON benchmarks after than and post the results would help confirm Not sure how to run those JSON benchmarks Check this guide:


https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#running-the-performance-test-suite

You can use the -b json switch at the end to limit the results to that module looks fine @WillAyd merge when good @ayappanec can you remove all occurrences of this macro instead of the empty define? [JSON_Benchmarks.txt](https://github.com/pandas-dev/pandas/files/4884421/JSON_Benchmarks.txt)
Attaching the JSON benchmarks results here @ayappanec can you run vs the master commit, e.g. https://pandas.pydata.org/docs/dev/development/contributing.html#running-the-performance-test-suite, you need `asv continuous` > @ayappanec can you run vs the master commit, e.g. https://pandas.pydata.org/docs/dev/development/contributing.html#running-the-performance-test-suite, you need `asv continuous`

Looks like hdf5 needs to be installed . I never encountered hdf5 before Installed HDF5 but now it fails with llvmlite build.

           raise RuntimeError("unsupported platform: %r" % (sys.platform,))
       RuntimeError: unsupported platform: 'aix6'

Looks like the asv benchmark testing needs some pre-requisites which has compilation issues in AIX . Any update on getting this merged ? thanks @ayappanec  @WillAyd @jreback Thanks for the support. 