Changes in gtest building after 8245610(remove in-tree copy on gtest)

[Igor Ignatyev]

Dear all,

The integration of 8245610[1-3] has slightly changed how to enable/disable hotspot Gtest test suite, which might affect some of you and require changes in your build/configure scripts. This email is to notify you about the possible impact and provide with the suggested changes for your `configure` option-set.

The patch removed the copy of Gtest framework which was located in `test/fmw/gtest`, now Gtest source code is to be stored separately and provided via `--with-gtest` configure options. Building the JDK[4] documentation has been updated to reflect this new dependency. The patch also deprecated `--(enable|disable)-hotspot-gtest` flags and changed defaults: unless `--with-gtest` points to the source code of Gtest, the test suite won't be built. jib profiles were updated to have Gtest as dependency, so `jib` automatically downloads Gtest source code and properly passes it to `--with-gtest` option.

The impacted people can be grouped by their needs to build Gtest test suite, and whether `jib` is used; the suggested changes for each group:
 - use `jib` and want Gtest test suite to be built: you are all set, no changes are required;
 - use `jib` and don't need Gtest test suite, i.e. you used to run `jib configure -- --disable-hotspot-gtest`: you need to replace `--disable-hotspot-gtest` by `--without-gtest`;
 - don't use `jib` and don't need Gtest test suite, i.e. you used to run `./configure --disable-hotspot-gtest`: you just need to remove `--disable-hotspot-gtest`;
 - don't use `jib` and want Gtest test suite to be built: you need to get Gtest source code and provide its path via `--with-gtest`. Running Tests[5] section gives suggestions on how the source code can be obtained.

Thanks,
-- Igor

[1] https://bugs.openjdk.java.net/browse/JDK-8245610
[2] https://mail.openjdk.java.net/pipermail/hotspot-dev/2020-May/041900.html
[3] https://hg.openjdk.java.net/jdk/jdk/rev/21c94b6f23e7
[4] http://hg.openjdk.java.net/jdk/jdk/raw-file/21c94b6f23e7/doc/building.html
[5] http://hg.openjdk.java.net/jdk/jdk/raw-file/21c94b6f23e7/doc/building.html#running-tests