PECL: Support versions in the extension name, e.g., 'proctitle-x.y.z'.

Clean up interpolated strings, wrap in { .. } when we have other content
in the string.
PECL: use '@' as the version specificator.

This should avoid issues with extensions that may contain a dash '-' in
their names.

Drop $version as it is not needed at the moment.
+github-php-extension-install, see README.