Build and test problems on Windows with gtest

[Matthias Perktold - ASA]

I wanted to create a pull request in OpenJDK, but I struggle to make the tests of tier 1 pass before making any changes.

At some point, I was able to run the tests of tier1, but got some failures (roughly about 60).

Then I noticed that the document says "Building of Hotspot Gtest suite requires the source code of Google Test framework".
So, I downloaded the source code and reconfigured the Toolchain, adding -with-gtest to the previously used config command, resulting in:
bash configure --with-boot-jdk=/cygdrive/c/jDev/java-x64/jdk-15.0.1+9/
  --with-jtreg=/cygdrive/c/jDev/jtreg/
  --with-gtest=/cygdrive/c/jDev/googletest-release-1.8.1/

But now I cannot run the tests anymore.
The test build exits with the message "ERROR: Build failed for target 'run-test-tier1' in configuration 'windows-x86_64-server-release' (exit code 2)".
Further up the logs, I found the following messages:

Creating support/test/lib-test/jtreg/native/bin/jvm-test-launcher.exe from 1 file(s)
make[3]: *** No rule to make target '/cygdrive/c/jdev/java-x64/jdk-15~2.1_9/bin//cygdrive/c/windows/system32/vcruntime140_1.dll', needed by '/cygdrive/c/users/matthiasp/git/jdk/build/windows-x86_64-server-release/images/test/hotspot/gtest/server/vcruntime140_1.dll'.  Stop.
make[3]: *** Waiting for unfinished jobs....
make[2]: *** [make/Main.gmk:675: test-image-hotspot-gtest] Error 2

Notice the double slash in the target path.
Also, it starts with the path to the boot JDK, but then it has "jdk-15~2.19" instead of "jdk-15.0.1+9".
Apparently, the paths got messed up here.

Do you have any idea what that could be, or what steps can help to solve this?
As you can see, I am on Windows with Cygwin.
Also, I installed Visual Studio 2019 Community Edition, adding "MSVC v142 - VS 2019 C++-ARM-Buildtools (v14.28)" to the core installation.

Thanks,
Matthias Perktold