Fix pkgconfig for openssl build

[malbarbo]

To avoid cache problems #1028 uses a temporary install directory for openssl. When the openssl install is completed the install dir is renamed. In doing so the pkgconfig files become invalid and libcurl-sys fails to find openssl and is built without https support. This fix the pkgconfig files. Fixes #1051.

[Diggsey]

@malbarbo Thanks, this looks important to fix ASAP

However, modifying pkgconfig directly with a regex seems a bit brittle, and if it breaks, we won't realise until too late.

What do you think of the following:

1. Cache the build directory instead of the install directory (the build directory can be safely renamed to retain the atomicity of the cache update, without breaking any paths) and run the make install command on every build (I would expect it to be relatively fast, but if that's not the case we can rethink).

2. Add a test to verify that curl was built with ssl support, whatever's easiest to implement, just so long as a regression here causes the build to fail.

[malbarbo]

@Diggsey I added a test to verify that rustup-init was built with ssl support.

I don't known exactly how to do the first point and I'm afraid that I cannot investigate it right now. Feel free to continue this fix.

Before a new release, consider fixes for #1047, #1055 and #1058

[malbarbo]

Docker was using the cached openssl build... Let's see how it goes now.

[malbarbo]

@Diggsey travis is green now.

[malbarbo]

@brson Considering that this is an important fix, you may be interest.

[jelford]

Fwiw a revert of #1028 would also be a reasonable option as a quick fix (it's strictly an optimization - when it works - so the only hit would be build time).

[malbarbo]

Closing in favor of #1065;