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

0 files changed, 0 insertions, 0 deletions