Allowing Amp to be compiled statically with x86_64-unknown-linux-musl

[cecton]

To make the compilation with x86_64-unknown-linux-musl works I need openssl-sys to be compiled with the feature "vendored" (see sfackler/rust-openssl#980 )

The feature flag is present in curl-sys and git2 but only starting from the version 0.8 of git2. So I upgraded git2 and fixed the tests.

Amp can now be compiled with MUSL.

[cecton]

hmmm... it worked for a smaller project ( ogham/exa#461 ) but for some reason here the binary I get is corrupted:

[0] [11:49:02] ~/r/amp vendored-openssl > ./target/x86_64-unknown-linux-musl/release/amp
Failed to execute process './target/x86_64-unknown-linux-musl/release/amp'. Reason:
The file './target/x86_64-unknown-linux-musl/release/amp' does not exist or could not be executed.
[125] [11:49:05] ~/r/amp vendored-openssl > ldd ./target/x86_64-unknown-linux-musl/release/amp
        linux-vdso.so.1 (0x00007ffe58de5000)
        libxcb.so.1 => /usr/lib/libxcb.so.1 (0x00007f00462f5000)
        libXau.so.6 => /usr/lib/libXau.so.6 (0x00007f00460f1000)
        libXdmcp.so.6 => /usr/lib/libXdmcp.so.6 (0x00007f0045eeb000)
        libc.so.6 => /usr/lib/libc.so.6 (0x00007f0045d27000)
        /lib/ld64.so.1 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f0046353000)

Well... This PR is still good and still a step forward to get it working at some point.

[jmacdonald]

Thanks for this, @cecton!

[cecton]

\o/ you're welcome