Add explicit dependency on extensions config

[007]

There isn't any dependency on openssl.ext even when it's used as the input for openssl --extfile. When I modified openssl.ext to test some modifications, test-keys didn't re-generate properly.

This PR fixes that by adding an explicit dependency each place it's used.

[CLAassistant]

All committers have signed the CLA.

[csstaub]

Neat, thanks @007!

[coveralls]

Coverage remained the same at 89.492% when pulling 396aebe on 007:patch-1 into da1f9e9 on square:master.