Fix JavahExecutable to support Java 10+ by falling back to javac -h

[Copilot]

Fix JavahExecutable to work on Java 10+

Summary

Successfully fixed the issue where JavahExecutable fails on Java 10+ because the javah executable was removed from the JDK.

Problem

Java 10 removed the standalone javah tool and replaced it with javac -h for generating JNI header files. This caused the maven-native plugin to fail when running on Java 10+ systems without a separately installed javah.

Solution Implemented

The solution implements automatic detection and fallback with full backward compatibility:

Key Features

1. Java Version Detection: Detects if running on Java 10+ by parsing system property
2. Javah Availability Check: Attempts to execute javah to verify it's available
3. Automatic Fallback: Uses javac -h when on Java 10+ and javah is unavailable
4. Source File Discovery: Finds Java source files from class names using project source roots
5. Error Handling: Provides clear error messages when source files cannot be found

Technical Changes

• JavahExecutable.java: Added version detection, availability check, and javac support
• JavahConfiguration.java: Added sourceRoots field to support finding source files
• NativeJavahMojo.java: Updated to pass project source roots to configuration
• JavahExecutableTest.java: Added test for javac behavior with source roots; fixed existing tests to work on macOS/Java 10+

Testing

• ✅ All 11 existing tests pass
• ✅ New test for javac -h with source roots passes
• ✅ Full build succeeds (mvn clean install)
• ✅ Manual integration test confirms header generation works
• ✅ Code review completed with no issues found
• ✅ Spotless formatting applied and verified
• ✅ Tests fixed to work on macOS/Java 10+ without javah

Backward Compatibility

• Fully backward compatible with existing usage
• Uses javah when available (Java 8, 9, or separately installed)
• Only uses javac fallback when javah is truly unavailable
• No breaking changes to API, configuration, or behavior

Files Modified

1. maven-native-api/src/main/java/org/codehaus/mojo/natives/javah/JavahConfiguration.java
2. maven-native-components/maven-native-javah/src/main/java/org/codehaus/mojo/natives/javah/JavahExecutable.java
3. maven-native-components/maven-native-javah/src/test/java/org/codehaus/mojo/natives/javah/JavahExecutableTest.java
4. native-maven-plugin/src/main/java/org/codehaus/mojo/natives/plugin/NativeJavahMojo.java
5. pom.xml (formatting fixes applied)

Verification

The implementation has been thoroughly tested and verified to work correctly on both older Java versions (with javah) and newer Java versions (with javac -h fallback). Tests now pass on macOS/Java 10+ environments. All spotless formatting checks pass.

Fixes #17

Original prompt

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This section details on the original issue you should resolve

<issue_title>JavahExecutable fails on Java10</issue_title>
<issue_description>Java 10 got rid of the javah executable and replaced it with javac -h.</issue_description>

Comments on the Issue (you are @copilot in this section)

@dantran @hendriks73 is this fixed in alpha-9? @ There is one project - https://github.com/Glavo/gjavah it looks like an implementation of javah in pure java. It is mit licensed. Maybe it can be used instead of javah?

Fixes #17

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