Steps to reproduce the issue

$ spack install expect
...
configure: error: could not find source file 'pty_.c'

Information on your system

• Spack: 0.15.4-1827-dcd514c321
• Python: 3.8.6
• Platform: darwin-catalina-ivybridge

Additional information

• spack-build-out.txt
• spack-build-env.txt

Seems related to http://expect.sourceforge.net/FAQ.html#q42

Don't see any special hacks in https://github.com/Homebrew/homebrew-core/blob/master/Formula/expect.rb

General information

• I have run spack debug report and reported the version of Spack/Python/Platform
• I have run spack maintainers <name-of-the-package> and @mentioned any maintainers
• I have uploaded the build log and environment files
• I have searched the issues of this repo and believe this is not a duplicate