Users can configure use of RPATH or RUNPATH (#9168)

Add a new entry in `config.yaml`:

    config:
        shared_linking: 'rpath'

If this variable is set to `rpath` (the default) Spack will set RPATH in ELF binaries. If set to `runpath` it will set RUNPATH.

Details:
* Spack cc wrapper explicitly adds `--disable-new-dtags` when linking
* cc wrapper also strips `--enable-new-dtags` from the compile line
    when disabling (and vice versa)
* We specifically do *not* add any dtags flags on macOS, which uses
    Mach-O binaries, not ELF, so there's no RUNPATH)

lib/spack/docs/config_yaml.rst

@@ -226,3 +226,24 @@ ccache`` to learn more about the default settings and how to change
 them). Please note that we currently disable ccache's ``hash_dir``
 feature to avoid an issue with the stage directory (see
 https://github.com/LLNL/spack/pull/3761#issuecomment-294352232).
+
+------------------
+``shared_linking``
+------------------
+
+Control whether Spack embeds ``RPATH`` or ``RUNPATH`` attributes in ELF binaries
+so that they can find their dependencies. Has no effect on macOS.
+Two options are allowed:
+
+ 1. ``rpath`` uses ``RPATH`` and forces the ``--disable-new-tags`` flag to be passed to the linker
+ 2. ``runpath`` uses ``RUNPATH`` and forces the ``--enable-new-tags`` flag to be passed to the linker
+
+``RPATH`` search paths have higher precedence than ``LD_LIBRARY_PATH``
+and ld.so will search for libraries in transitive ``RPATHs`` of
+parent objects.
+
+``RUNPATH`` search paths have lower precedence than ``LD_LIBRARY_PATH``,
+and ld.so will ONLY search for dependencies in the ``RUNPATH`` of
+the loading object.
+
+DO NOT MIX the two options within the same install tree.