bpo-38634: Allow non-apple build to cope with libedit (GH-16986)

 bpo-38634: Allow non-apple build to cope with libedit (GH-16986)

The readline module now detects if Python is linked to libedit at runtime
on all platforms.  Previously, the check was only done on macOS.

If Python is used as a library by a binary linking to libedit, the linker
resolves the rl_initialize symbol required by the readline module against
libedit instead of libreadline, which leads to a segfault.

Take advantage of the existing supporting code to have readline module being
compatible with both situations.