Only build static executable if musl toolchain is installed (#83)

This improves the development experience for (WSL) Linux users.
They can now run "./gradlew buildNative" without having a musl toolchain installed.
In this case, only the dynamically linked executable will be built.

buildSrc/src/main/kotlin/BuildInfo.kt

@@ -95,6 +95,11 @@ open class BuildInfo(project: Project) {
     java.lang.Boolean.getBoolean("releaseBuild")
   }
 
+  val hasMuslToolchain: Boolean by lazy {
+    // see "install musl" in .circleci/jobs/BuildNativeJob.pkl
+    File(System.getProperty("user.home"), "staticdeps/bin/x86_64-linux-musl-gcc").exists()
+  }
+
   val os: org.gradle.internal.os.OperatingSystem by lazy {
     org.gradle.internal.os.OperatingSystem.current()
   }