OpenJDK builds have always required freetype which is only
available as a system library on Linux.

On Windows and Mac developers need to build freetype separately
and then point to that pre-built freetype as part of the configure process.
This then gets bundled into the JDK so that it is available at run time.

The way linking works on Mac the libfontmanager.dylib this requires
an extra step (only very recently fixed, see https://bugs.openjdk.java.net/browse/JDK-8139653)

And once you are shipping a library like freetype, you want to keep
it up to date. Once build infrastructure needs to be updated to get
this updated version it is getting harder to do that, without much
advantage .. the size of the freetype sources and the time to build
it are both insignificant. So updating the version in the source is
much easier and transparent.

None of this need require that a Linux (or Solaris) build should use
the JDK version. Those can continue to use the system version if so
preferred.

And it will make life a lot easier for people building OpenJDK on
Windows and Mac .. it has been a pain point for a long time.