BREAKING CHANGE: IncludeCustomExtension behavior changes

[Magnus Ihse Bursie]

Some details on how the IncludeCustomExtension hook is called will 
change when JDK-8348190[1] is integrated. The PR is currently open for 
reviewing on GitHub[2].

The big benefit of this change is that now the hooks will be 
automatically generated, so it will be possible to add hooks to each and 
every makefile without having to make any changes in the open JDK source 
code. However, as a result of this, the hooks will always only appear 
strictly before or after a makefile. While many of the current hooks are 
positioned closed to the top or bottom of a file, not all are, and this 
might change the behavior enough to render a pre-existing hook broken 
and in need of fixing to keep working.

Furthermore, the names passed to IncludeCustomExtension have been 
standardized to always end in -pre or -post, which was often, but not 
always, the case before. Rename your hooks accordingly.

/Magnus

[1] https://bugs.openjdk.org/browse/JDK-8348190
[2] https://github.com/openjdk/jdk/pull/23399