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).
a8706cc89b