lexer: Change the pipeline to allow more flexible use of macros.

lexer: Change the pipeline to allow more flexible use of macros.

Frans Houweling reported that a use of macros in the following way:

    DEFINE !dir() "/directory/to/my/work" !ENDDEFINE.
    GET FILE=!dir + "/filename.sav".

did not work properly with the newly implemented PSPP macro facility.
Indeed, PSPP has until now implemented string concatenation early in the
lexical pipeline, before macro expansion, so that the above could
not work.  This commit reworks it so that string concatenation happens
as the last stage in lexical analysis.  It allows the above syntax
to work as expected.