Add a translateOnly mode that skips compilation-dependent tasks.

Related to j2objc-contrib#345; Fixes j2objc-contrib#347

FAQ.md

@@ -223,7 +223,7 @@ and building the J2ObjC source:
 3. Configure j2objcConfig in `shared/build.gradle` so CycleFinder uses the annotated J2ObjC
 source and whitelist. Note how this gives and expected cycles of zero.
 
-```
+```gradle
     // File: shared/build.gradle
     j2objcConfig {
         cycleFinderArgs '--whitelist', 'J2OBJC_REPO/jre_emul/cycle_whitelist.txt'
@@ -244,10 +244,17 @@ Development is officially supported only on Mac OS X. This matches the
 In practice, the j2objcCycleFinder and j2objcTranslate tasks may work, though it is
 strongly suggested to use this for experimentation only and do all proper development
 on OS X. The team will reject any bugs related to systems that don't meet the
-[minimum requirements](README.md#minimum-requirements).
-
-To experiment on Windows (skipping the unsupported tasks), the following may work:
+[minimum requirements](README.md#minimum-requirements). You can attempt just
+j2objcCycleFinder and j2objcTranslate tasks with:
 
-    ./gradlew shared:j2objcAssembleSource shared:j2objcAssembleResources
+```gradle
+    // File: shared/build.gradle
+    j2objcConfig {
+        ...
+        translateOnlyMode = true
+        ...
+        finalConfigure()
+    }
+```
 
 Please note that this will not build the packed libraries or update your Xcode project.

src/main/groovy/com/github/j2objccontrib/j2objcgradle/J2objcConfig.groovy

@@ -212,6 +212,12 @@ class J2objcConfig {
         appendArgs(this.translateSourcepaths, 'translateSourcepaths', translateSourcepaths)
     }
 
+    /**
+     * True iff only translation (and cycle finding, if applicable) should be attempted,
+     * skipping all compilation, linking, and testing tasks.
+     */
+    boolean translateOnlyMode = false
+
 
     // Do not use groovydoc, this option should remain undocumented.
     // WARNING: Do not use this unless you know what you are doing.
@@ -530,6 +536,18 @@ class J2objcConfig {
                 }
             }
         }
+
+        // Support translation-only mode.
+        if (translateOnlyMode) {
+            project.tasks.all { Task task ->
+                // First pattern matches all native-compilation tasks.
+                // Second pattern matches plugin-specific tasks beyond translation.
+                if ((task.name =~ /^.*((J|j)2objc(Executable|StaticLibrary|SharedLibrary|Objc))$/).matches() ||
+                    (task.name =~ /^j2objc(Assemble|PackLibraries|Test)(Debug|Release)$/).matches()) {
+                    task.enabled = false
+                }
+            }
+        }
     }
     boolean isFinalConfigured() {
         return finalConfigured

src/main/groovy/com/github/j2objccontrib/j2objcgradle/J2objcPlugin.groovy

@@ -26,7 +26,6 @@ import com.github.j2objccontrib.j2objcgradle.tasks.TranslateTask
 import com.github.j2objccontrib.j2objcgradle.tasks.Utils
 import com.github.j2objccontrib.j2objcgradle.tasks.XcodeTask
 import org.gradle.api.DefaultTask
-import org.gradle.api.InvalidUserDataException
 import org.gradle.api.Plugin
 import org.gradle.api.Project
 import org.gradle.api.Task
@@ -112,6 +111,10 @@ class J2objcPlugin implements Plugin<Project> {
                 description "Run the cycle_finder tool on all Java source files"
                 enabled false
             }
+
+            // NOTE: When adding new tasks, consider whether they fall under 'translate-only' mode, and
+            // filter them in J2objcConfig.finalConfigure otherwise.
+
             // Note the '(debug|release)TestJ2objcExecutable' tasks are dynamically created by the Objective-C plugin.
             // It is specified by the testJ2objc native component in NativeCompilation.groovy.
             // TODO: copy and run debug and release tests within j2objcTestContent at the