cmake.xml: improve the generator to produce optimized regexes

Split strings containing `<variadic_part>` by `_` char and build
a tree of name components. Having this tree it's possible to reduce
common parts of regular expressions and transform the remainder into
an alt group. For example, the following list of variables to match

    CMAKE_<some>_ONE
    CMAKE_<some>_TWO
    CMAKE_<some>_BLAH_SATU
    CMAKE_<some>_BLAH_DUA

give the following regex to match:

    CMAKE_&var_ref;_(BLAH_(SATU|DUA)|ONE|TWO)

instead of naive way used before:

    CMAKE_&var_ref;_ONE|CMAKE_&var_ref;_TWO|CMAKE_&var_ref;_BLAH_SATU|CMAKE_&var_ref;_BLAH_DUA

Also, add some features missed before.