Add optional SKIKO_VSBT_PATH env. variable

Sometimes Gradle C++ plugin fails to find Visual Studio Build Tools
automatically (see https://github.com/gradle/gradle-native/issues/617).

This commit adds optional environment variable for providing
VSBT path manually.

README.md

@@ -6,3 +6,26 @@
 [here](https://bintray.com/beta/#/jetbrains/skija/Skia?tab=files))
 * Set JAVA_HOME to location of JDK, at least version 11
 * `cd skiko && ./gradlew publishToMavenLocal` will build the artefact and publish it to local Maven repo
+
+
+#### Building on Windows
+
+##### Using Visual Studio C++
+* Install [Visual Studio Build Tools](https://visualstudio.microsoft.com/visual-cpp-build-tools/) or
+[Visual Studio C++](https://visualstudio.microsoft.com/vs/features/cplusplus/).
+* If Gradle fails to find Visual C++ toolchain:
+```
+Execution failed for task ':compileDebugWindowsCpp'.
+> No tool chain is available to build C++ for host operating system 'Windows 10' architecture 'x86-64':
+    - Tool chain 'visualCpp' (Visual Studio):
+        - Could not locate a Visual Studio installation, using the command line tool, Windows registry or system path.
+```
+set `SKIKO_VSBT_PATH` environment variable to the location of installed Visual Studio Build Tools. 
+This could be done in the UI:
+```
+Control Panel|All Control Panel Items|System|Advanced system settings|Environment variables
+```
+or by running `cmd` as administrator:
+```
+setx /M SKIKO_VSBT_PATH "C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools"
+```
\ No newline at end of file

skiko/build.gradle.kts

@@ -313,6 +313,16 @@ library {
              include("**.o")
         })
     }
+
+    toolChains {
+        withType(VisualCpp::class.java) {
+            // In some cases Gradle is unable to find VC++ toolchain
+            // https://github.com/gradle/gradle-native/issues/617
+            properties.visualStudioBuildToolsDir?.let {
+                setInstallDir(it)
+            }
+        }
+    }
 }
 
 val skikoJvmJar: Provider<Jar> by tasks.registering(Jar::class) {
@@ -464,6 +474,9 @@ class SkikoProperties(private val myProject: Project) {
         get() = (myProject.property("dependencies.skia.release.pattern") as String)
                 .replace("<TARGET_OS>", target)
 
+    val visualStudioBuildToolsDir: File?
+        get() = System.getenv()["SKIKO_VSBT_PATH"]?.let { File(it) }?.takeIf { it.isDirectory }
+
     val dependenciesDir: File
         get() = myProject.rootProject.projectDir.resolve("dependencies")
 }