puma_http11.c:203:22: note: include the header <ctype.h> or explicitly provide a declaration for 'isspace'
Are you facing this error recently while trying to install puma gem 4.3.5 on Mac OS X?
This issue is reported on Puma issue tracker here and fixed in version 4.3.6 and in the latest 5.0.0 release.
But if you want to fix it without updating the Puma gem version from 4.3.5, you can use run following command to update your Bundler configuration.
bundle config build.puma --with-cflags="-Wno-error=implicit-function-declaration"
bundle install will run successfully after this.
Even better is to just update Puma to latest version which also fixes this issue.