The build of WebKit on Windows fails with the messages:

  CMake Error at CMakeLists.txt:13 (project):
    Generator

      Visual Studio 15 2017 Win64

    could not find any instance of Visual Studio.

SYSTEM / OS / JAVA RUNTIME INFORMATION

Microsoft Windows 10 Pro version 10.0.19042
Microsoft Visual Studio 2019 version 16.9.4

Cygwin 3.2.0
  $ uname -srm
  CYGWIN_NT-10.0 3.2.0(0.340/5/3) x86_64

CMake 3.20.0
  $ cmake --version
  cmake version 3.20.0

Oracle OpenJDK 15.0.2
  $ java --version
  openjdk 15.0.2 2021-01-19
  OpenJDK Runtime Environment (build 15.0.2+7-27)
  OpenJDK 64-Bit Server VM (build 15.0.2+7-27, mixed mode, sharing)

Apache Ant 1.10.9
  $ ant -version
  Apache Ant(TM) version 1.10.9 compiled on September 27 2020

Gradle 6.3
  $ gradle --version

  ------------------------------------------------------------
  Gradle 6.3
  ------------------------------------------------------------

  Build time: 2020-03-24 19:52:07 UTC
  Revision: bacd40b727b0130eeac8855ae3f9fd9a0b207c60

  Kotlin: 1.3.70
  Groovy: 2.5.10
  Ant: Apache Ant(TM) version 1.10.7 compiled on September 1 2019
  JVM: 15.0.2 (Oracle Corporation 15.0.2+7-27)
  OS: Windows 10 10.0 amd64

STEPS TO REPRODUCE

Follow the instructions for building JavaFX on Windows found here:

Building OpenJFX
https://wiki.openjdk.java.net/display/OpenJFX/Building+OpenJFX

Run a build that includes WebKit. For example, I ran:

  $ gradle -PCOMPILE_WEBKIT=true sdk

EXPECTED RESULTS

The build succeeds:

  BUILD SUCCESSFUL in 1h 10m 37s
  140 actionable tasks: 140 executed

ACTUAL RESULT

The build fails:

  > Task :web:compileNativeWin
  + cmake ...
  CMake Error at CMakeLists.txt:13 (project):
    Generator

      Visual Studio 15 2017 Win64

    could not find any instance of Visual Studio.

  -- Configuring incomplete, errors occurred!
  See also "C:/cygwin64/home/john/src/jfx/
    modules/javafx.web/build/win/Release/CMakeFiles/CMakeOutput.log".

  > Task :web:compileNativeWin FAILED

  FAILURE: Build failed with an exception.

  * Where:
  Build file 'C:\cygwin64\home\john\src\jfx\build.gradle' line: 3491

  * What went wrong:
  Execution failed for task ':web:compileNativeWin'.
  > Process 'command 'perl'' finished with non-zero exit value 1

  ...

  BUILD FAILED in 4m 20s
  55 actionable tasks: 55 executed

SOURCE CODE FOR AN EXECUTABLE TEST CASE

None.

WORKAROUND

I worked around the problem by patching the file:

  modules/javafx.web/src/main/native/Tools/Scripts/webkitdirs.pm

as shown in the attached file called 'webkitdirs.pm.patch'.