[Issue 18649] curl on Ubuntu 18.04 depends on libcurl4, .deb installer depends on libcurl3

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18649

--- Comment #6 from Seb <greensunny12 at gmail.com> ---
An attempt at fixing it by simply making libcurl optional (it's only needed for
std.net.curl) https://github.com/dlang/installer/pull/314

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