document usage of compiler flags properties

lib/spack/docs/packaging_guide.rst

@@ -1831,6 +1831,25 @@ successfully find ``libdwarf.h`` and ``libdwarf.so``, without the
 packager having to provide ``--with-libdwarf=/path/to/libdwarf`` on
 the command line.
 
+Compiler flags
+~~~~~~~~~~~~~~
+In rare circumstances such as compiling and running small unit tests, a package
+developer may need to know what are the appropriate compiler flags to enable
+features like ``OpenMP``, ``c++11``, ``c++14`` and alike. To that end the
+compiler classes in ``spack`` implement the following _properties_ :
+``openmp_flag``, ``cxx11_flag``, ``cxx14_flag``, which can be accessed in a
+package by ``self.compiler.cxx11_flag`` and alike. Note that the implementation
+is such that if a given compiler version does not support this feature, an
+error will be produced. Therefore package developers can also use these properties
+to assert that a compiler supports the requested feature. This is handy when a
+package supports additional variants like
+
+.. code-block:: python
+
+   variant('openmp', default=True, description="Enable OpenMP support.")
+
+
+
 Message Parsing Interface (MPI)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 It is common for high performance computing software/packages to use ``MPI``.