pchreuse: defer dependency addition until generation time

Note that this does lose the backtrace information. It also, however,
makes `PRECOMPILE_HEADERS_REUSE_FROM` a non-magic property now.
Previously, appending to the property would miss all of its behaviors
(or leave existing setup in-place which is no longer relevant
post-append). Rather than making a policy to block appending, just defer
all logic to generation time.