Make CFG_LIBDIR configurable

[fabiand]

Currently CFG_LIBDIR is hard coded. it would be nice if this path could be adjusted using some configure arg.

[brson]

I'm curious what this would be used for. CFG_LIBDIR exists to make dynamic loading slightly less bad on windows, by placing executables and libraries in the same directory.

[brson]

As you mentioned in #5219, distros with multiarch support often put their libs in places like /usr/lib64. I understand the use case now and agree it should be configurable. Let's use whatever configure flag is standard.

[graydon]

assigning bug; change assignment if you disagree

[catamorphism]

I don't think this should block 0.7 -- re-milestone if you disagree, @graydon

[jhasse]

Hi everyone! I tried to fix this with this commit.

The problem with the current implementation in configure: It sets CFG_LIBDIR to "lib" or "bin". But according to http://www.gnu.org/prep/standards/html_node/Directory-Variables.html libdir should always be an absolute path e.g. /usr/local/lib.

Also it overrides the value which could have been set by the user, because it is set after the call to valopts.

My patch fixes these issue and also make CFG_LIBDIR configurable: I introduced a new variable CFG_LIBDIR_RELATIVE which has the value CFG_LIBDIR has now (lib or bin). CFG_LIBDIR is then set to the correct absolute value.

After my patch, let's say someone sets libdir without changing the prefix:

$ ./configure --libdir=/usr/lib64
[...]
configure: CFG_PREFIX           := /usr/local
[...]
configure: CFG_LIBDIR           := /usr/lib64
configure: error: libdir must begin with the prefix. Use --prefix to set it accordingly.

This is because the script is unsure what the name of the library directory (relative to the prefix) is. He also has to change the prefix:

$ ./configure --libdir=/opt/lib64 --prefix=/opt
[...]
configure: CFG_PREFIX           := /opt
[...]
configure: CFG_LIBDIR           := /opt/lib64
configure: CFG_LIBDIR_RELATIVE  := lib64
[...]

The CFG_LIBDIR_RELATIVE variable then holds lib64 which is what will be used in several places by rust. For example rustpkg places libaries in a folder called lib64 instead of lib. Also this will result in stage1/lib64/ during the build phase.

If I run configure on my Windows machine without any arguments this will result in the following:

$ ./configure
[...]
configure: CFG_PREFIX           := /usr/local
[...]
configure: CFG_BUILD            := i686-pc-mingw32
[...]
configure: CFG_LIBDIR           := /usr/local/bin
configure: CFG_LIBDIR_RELATIVE  := bin
[...]

Everything as it should be ;)

What do you think?