. Synchronise with jdk13 Makefile as much as possible to reduce
maintenance. The main changes are:
. Use 'p' for the patchset version, not 'b' and refer to the
patchset internally with JDK_PATCHSET_VERSION rather than
. Add the NATIVE_BOOTSTRAP feature and synchronise DEPENDS lists
as much as possible.
. Add the extra pre-build checks to try and avoid build errors.
. There is also a very minor amount of whitespace diff reduction (this
should have gone in a separate commit). Last hunk of the diff.