BLD: Avoid "visibility attribute not supported" warning (gh-16288)
BLD: Avoid "visibility attribute not supported" warning (gh-16288)

I found that when building the latest master branch on Cygwin, while testing #16246, that thousands of warnings were generated at build time like:

numpy/core/src/npysort/binsearch.c.src: In function ‘binsearch_left_bool’:
numpy/core/src/npysort/binsearch.c.src:82:1: warning: visibility attribute not supported in this configuration; ignored [-Wattributes]
Granted this is just a warning, so I don't think it's a serious issue.

It seems the test that was supposed to check for __attribute__ support was not working as expected. The #pragmas only take effect if I provide a function body--they are ignored for bare declarations. I don't know if that's by intent, or if it's a GCC issue. For reference:

$ gcc --version
gcc (GCC) 7.4.0
pdebuyl/numpy,pdebuyl/numpy,madphysicist/numpy,charris/numpy,anntzer/numpy,simongibbons/numpy,mhvk/numpy,madphysicist/numpy,simongibbons/numpy,seberg/numpy,charris/numpy,anntzer/numpy,pbrod/numpy,rgommers/numpy,pdebuyl/numpy,numpy/numpy,numpy/numpy,mattip/numpy,anntzer/numpy,abalkin/numpy,numpy/numpy,seberg/numpy,mattip/numpy,seberg/numpy,madphysicist/numpy,grlee77/numpy,jakirkham/numpy,madphysicist/numpy,endolith/numpy,seberg/numpy,rgommers/numpy,mhvk/numpy,pdebuyl/numpy,mhvk/numpy,mattip/numpy,mattip/numpy,jakirkham/numpy,grlee77/numpy,charris/numpy,madphysicist/numpy,abalkin/numpy,numpy/numpy,abalkin/numpy,mhvk/numpy,anntzer/numpy,jakirkham/numpy,jakirkham/numpy,simongibbons/numpy,pbrod/numpy,jakirkham/numpy,simongibbons/numpy,pbrod/numpy,grlee77/numpy,endolith/numpy,rgommers/numpy,rgommers/numpy,mhvk/numpy,endolith/numpy,endolith/numpy,grlee77/numpy,pbrod/numpy,pbrod/numpy,simongibbons/numpy,grlee77/numpy,charris/numpy