Allow compiler wrapper to modify environment (#2275)

* Allow compiler wrapper to modify environment

This adds the ability to set environment variables in the compiler
wrappers. These are specified as part of the compilers.yaml config.
The user may also specify RPATHs in compilers.yaml that should be
added.

* Minor doc tweak

lib/spack/docs/basic_usage.rst

@@ -672,6 +672,35 @@ in GNU Autotools. If all flags are set, the order is
 ``$cppflags $cflags|$cxxflags $ldflags <command> $ldlibs`` for C and C++ and
 ``$fflags $cppflags $ldflags <command> $ldlibs`` for Fortran.
 
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Compiler environment variables and additional RPATHs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In the exceptional case a compiler requires setting special environment
+variables, like an explicit library load path. These can bet set in an
+extra section in the compiler configuration.  The user can also specify
+additional ``RPATHs`` that the compiler will add to all executables
+generated by that compiler.  This is useful for forcing certain compilers
+to RPATH their own runtime libraries, so that executables will run
+without the need to set ``LD_LIBRARY_PATH``.
+
+.. code-block:: yaml
+
+  compilers:
+    - compiler:
+        spec: gcc@4.9.3
+        paths:
+          cc: /opt/gcc/bin/gcc
+          c++: /opt/gcc/bin/g++
+          f77: /opt/gcc/bin/gfortran
+          fc: /opt/gcc/bin/gfortran
+        environment:
+          set:
+            LD_LIBRARY_PATH : /opt/gcc/lib
+        extra_rpaths:
+        - /path/to/some/compiler/runtime/directory
+        - /path/to/some/other/compiler/runtime/directory
+
 ^^^^^^^^^^^^^^^^^^^^^^^
 Architecture specifiers
 ^^^^^^^^^^^^^^^^^^^^^^^