When cxxflags is globally set to an empty value, the getOption magic
that appends module values to global values for cxxflags causes it the
result to equal ' ' (i.e. one space). This is because the space is a
separator between two empty values.

This causes code testing against cxxflags to think it's actually been
set to a value and to add it in the cmake calls.

I fix this and add a test case, but also add some insurance by trimming
leading/trailing white space so that the existing check for empty
cxxflags would have had a chance to catch this.

Differential Revision: https://phabricator.kde.org/D18165