Help: Clarify cmake-buildsystem(7) compile properties section names
In !9454 (merged) we labeled the sections documenting COMPILE_*
properties as "build properties". All the properties it documents are
about compilation steps before linking, including the Autogen features
for compiling Qt metadata, so rename the sections as "compile properties".
This is also consistent with the name of the $<COMPILE_ONLY:...>
generator expression.