When I'm writing a user defined rule I should use ptxdist variables instead absolute paths or values.

There a lot of variables available in a project (e.a. PTXDIST_CROSS_CPPFLAGS, PTXDIST_CROSS_LDFLAGS, PTXDIST_PLATFORMCONFIG, PTXCONF_GNU_TARGET, ....), but from where do I know the names of these variables?
How can I obtain a list about available variables for a project?

Best regards,
Jan-Marc Stranz.