configure on macOS complains "integer expression expected"

[DougBurke]

I am guessing this is not a serious issue, but let's not forget about it:

On Linux, python setup.py develop produces

...
checking for incorporation of thread support... no
checking for gtk... no
checking linux-gnu configuration... none
configure: creating ./config.status
config.status: creating Makefile
config.status: creating xpa.pc
...

whereas on macOS you see something like (e.g. #78 (comment)):

...
checking for incorporation of thread support... no
checking for gtk... no
checking darwin18.7.0 configuration... ./configure: line 7817: test: version: integer expression expected
configure: creating ./config.status
config.status: creating Makefile
config.status: creating xpa.pc
...

[ericmandel]

I see that the xpa folder in cextern/xpa is pretty old. The configure script has a line of code that is causing the problem but is not in the new configure script. There also are a number of minor changes to the source code, presumably to avoid compiler warnings. At some point, we should update the xpa package to whatever is current.