auto merge of #14832 : alexcrichton/rust/no-rpath, r=brson

This commit disables rustc's emission of rpath attributes into dynamic libraries
and executables by default. The functionality is still preserved, but it must
now be manually enabled via a `-C rpath` flag.

This involved a few changes to the local build system:

* --disable-rpath is now the default configure option
* Makefiles now prefer our own LD_LIBRARY_PATH over the user's LD_LIBRARY_PATH
  in order to support building rust with rust already installed.
* The compiletest program was taught to correctly pass through the aux dir as a
  component of LD_LIBRARY_PATH in more situations.

The major impact of this change is that neither rustdoc nor rustc will work
out-of-the-box in all situations because they are dynamically linked. It must be
arranged to ensure that the libraries of a rust installation are part of the
LD_LIBRARY_PATH. The default installation paths for all platforms ensure this,
but if an installation is in a nonstandard location, then configuration may be
necessary.

Additionally, for all developers of rustc, it will no longer be possible to run
$target/stageN/bin/rustc out-of-the-box. The old behavior can be regained
through the `--enable-rpath` option to the configure script.

This change brings linux/mac installations in line with windows installations
where rpath is not possible.

Closes #11747
[breaking-change]

man/rustc.1

@@ -138,8 +138,8 @@ A space-separated list of arguments to pass through to LLVM.
 If specified, the compiler will save more files (.bc, .o, .no-opt.bc) generated
 throughout compilation in the output directory.
 .TP
-\fBno-rpath\fR
-If specified, then the rpath value for dynamic libraries will not be set in
+\fBrpath\fR
+If specified, then the rpath value for dynamic libraries will be set in
 either dynamic library or executable outputs.
 .TP
 \fBno-prepopulate-passes\fR