Add staticssl dub config to link to openssl statically (linux only)

[skoppe]

dub test was failing on my pc. I tried to get it going but could only do it by going through the follow unusual sequence:

• removing sourcePaths from the unittest config
• run dub test, which failed
• reintroduce sourcePaths to the unittest config
• run dub test, which passed

It is unclear to me why this is the case, but I had to go through the hoops on every test run...

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Aside from that it could perhaps use some example how to statically link on e.g. alpine. Lets first review this though.

[ikod]

Thanks, @skoppe

[ikod]

dub test was failing on my pc. I tried to get it going but could only do it by going through the follow unusual sequence:

* removing `sourcePaths` from the unittest config

* run `dub test`, which failed

* reintroduce `sourcePaths` to the unittest config

* run `dub test`, which passed

It is unclear to me why this is the case, but I had to go through the hoops on every test run...

Thanks!
This is something introduced recently in compiler or in dub. Will try to fix it when will have some more time.