Bypass sigchild detection if phpinfo is not available

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | composer/composer#2711
| License       | MIT
| Doc PR        | none

This fixes composer/composer#2711, i.e. anyone disabling phpinfo for security (by obscurity, but whatever) reasons can not use the Process class anymore.