Look for javac instead of java

There are some situations where using `bin/java` as a heuristic
to find JAVA_HOME within a JDK package can result in mistakenly
finding a JRE root instead. Looking for `bin/javac` fixes this
since JREs do not have `javac`.

This should fix palantir#97

gradle-jdks/src/main/java/com/palantir/gradle/jdks/JdkManager.java

@@ -194,7 +194,7 @@ private Path findJavaHome(Path temporaryJdkPath) {
             return files.filter(file -> Files.isRegularFile(file)
                             // macos JDKs have a `bin/java` symlink to `Contents/Home/bin/java`
                             && !Files.isSymbolicLink(file)
-                            && file.endsWith(Paths.get("bin", SystemTools.java())))
+                            && file.endsWith(Paths.get("bin", SystemTools.javac())))
                     .findFirst()
                     // JAVA_HOME/bin/java
                     .orElseThrow(() -> new RuntimeException("Failed to find java home in " + temporaryJdkPath))

gradle-jdks/src/main/java/com/palantir/gradle/jdks/SystemTools.java

@@ -26,6 +26,13 @@ static String java() {
         return "java";
     }
 
+    static String javac() {
+        if (Os.current() == Os.WINDOWS) {
+            return "javac.exe";
+        }
+        return "javac";
+    }
+
     static String keytool() {
         if (Os.current() == Os.WINDOWS) {
             return "keytool.exe";