CMakePackage: convert variants to CMake arguments (#14376)

Add a 'define_from_variant` helper function to CMake-based Spack
packages to convert package variants into CMake arguments. For
example:

  args.append('-DFOO=%s' % ('ON' if '+foo' in self.spec else 'OFF'))

can be replaced with:

  args.append(self.define_from_variant('foo'))

The following conversions are handled automatically:

* Flag variants will be converted to CMake booleans
* Multivalued variants will be converted to semicolon-separated strings
* Other variant values are converted to CMake string arguments

This also adds a 'define' helper method to convert any variable to
a CMake argument. It has the same conversion rules as
'define_from_variant' (but operates directly on values rather than
requiring the user to supply the name of a package variant).

lib/spack/docs/build_systems/cmakepackage.rst

@@ -128,17 +128,20 @@ Adding flags to cmake
 ^^^^^^^^^^^^^^^^^^^^^
 
 To add additional flags to the ``cmake`` call, simply override the
-``cmake_args`` function:
+``cmake_args`` function. The following example defines values for the flags
+``WHATEVER``, ``ENABLE_BROKEN_FEATURE``, ``DETECT_HDF5``, and ``THREADS`` with
+and without the :py:meth:`~.CMakePackage.define` and
+:py:meth:`~.CMakePackage.define_from_variant` helper functions:
 
 .. code-block:: python
 
    def cmake_args(self):
-       args = []
-
-       if '+hdf5' in self.spec:
-           args.append('-DDETECT_HDF5=ON')
-       else:
-           args.append('-DDETECT_HDF5=OFF')
+       args = [
+           '-DWHATEVER:STRING=somevalue',
+           self.define('ENABLE_BROKEN_FEATURE', False),
+           self.define_from_variant('DETECT_HDF5', 'hdf5'),
+           self.define_from_variant('THREADS'), # True if +threads
+       ]
 
        return args