CLASSPATH is supposed to show where to find the classes needed by Java
programs running at build time. The -cp switch to javac tells where to
find classes referenced by the code being compiled. These are
different things. (But it doesn't seem to have mattered much in our
build system.) So use T_CP instead, named in the same fashion as
T_CXXFLAGS etc.

But... for some reason this change, which as such should be just more
or less cosmetic, also fixes a build problem in the "scripting" module
on Windows, seen by Noel Grandin
(http://comments.gmane.org/gmane.comp.documentfoundation.libreoffice.devel/19016
) and me.