Previously we rejected all preprocessor definition values containing
spaces for the VS6 IDE generator.  In fact VS6 does support spaces but
not in combination with '"', '$', or ';', and only if we use the sytnax
'-DNAME="value with spaces"' instead of '-D"NAME=value with spaces"'.
Now we support all definition values that do not have one of these
invalid pairs.  See issue #8779.