Fix installing into --libdir other than .../lib

[anguslees]

Currently, --libdir path rewriting happens twice:

1. 'prepare' make target places files into file tree and builds tarball
2. rust-installer copies from tarball paths into final filesystem

Both these steps are attempting to handle --libdir. The installer assumes
lib paths are 'lib/' (the default) and when they aren't you can end up with
duplicated $(LIBDIR_RELATIVE)/$(LIBDIR_RELATIVE)/ paths.

This patch addresses this by making step (1) always install into lib/ and
performing --libdir handling only during step (2).

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

r? @brson

I haven't touched this in awhile...

[brson]

Yes, I should review this!

[anguslees]

@brson (bump)

[steveklabnik]

Another bump

[brson]

I've been willfully ignoring this PR because the ramifications around handling libdir in Rust are more complex than I want to think about. I'm very sorry.

Some observations:

• This patch retains the 'internal' file hierarchy customized by CFG_LIBDIR_RELATIVE, then undoes that customization when preparing the install image. If we're going in this direction, I'd prefer to keep a consistent file system structure for the build (i.e. always use 'lib' as the directory name), and only frob libdir/mandir, etc. during the final installation (sink it into rust-installer).
• On windows we use 'bin' for the libdir so that windows' dynamic linker can more easily resolve the dylibs. This patch will break installation on windows by not accounting for that. In a world where we use a consistent directory layout for the build, I might expect rust-installer to automatically install 'lib' files to 'bin' on windows.
• rustc does not work correctly with unexpected values of 'lib' and 'bin' because it calculates their location from the relative path from the rustc binary.

So I'd say the way I'd want to move forward on this is:

• Teach rust-installer to automatically install the lib folder to bin on windows, possibly only when configured a certain way since this is weird rust-specific behavior and rust-installer is nominally a generic installer.
• Remove CFG_LIBDIR_RELATIVE from the makefiles completely so all lib builds go in lib/.

[brson]

Ah, hm. The reason this may leave the custom heirarchy around for the build is probably because we use --libdir to statically alter rustcs sysroot calculation (iow if you set--libdir=foorustc looks for libs in../fooinstead of../lib`. I don't like this much because then it produces an installer that is customized to the particular system you are building on.

[alexcrichton]

Closing due to inactivity, but feel free to reopen with comments addressed!