Build: Update javah target to work with JDK 10+

javah is no longer a thing, but 'javac -h' can accomplish the same task.
diff --git a/java/CMakeLists.txt b/java/CMakeLists.txt
index d51ada7..3d863bd 100644
--- a/java/CMakeLists.txt
+++ b/java/CMakeLists.txt
@@ -58,11 +58,21 @@
 add_custom_target(javadoc COMMAND
   javadoc -notimestamp -d ${CMAKE_CURRENT_SOURCE_DIR}/doc -sourcepath ${CMAKE_CURRENT_SOURCE_DIR} org.libjpegturbo.turbojpeg)
 set(JAVACLASSPATH ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_FILES_DIRECTORY}/turbojpeg-java.dir)
-add_custom_target(javah
-  COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJ
-  COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJCompressor
-  COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJDecompressor
-  COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJTransformer)
+if(Java_VERSION_MAJOR GREATER 9)
+  add_custom_target(javah
+    COMMAND javac -h ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH}
+      -d ${CMAKE_CURRENT_BINARY_DIR}/__unused
+      ${CMAKE_CURRENT_SOURCE_DIR}/org/libjpegturbo/turbojpeg/TJ.java
+      ${CMAKE_CURRENT_SOURCE_DIR}/org/libjpegturbo/turbojpeg/TJCompressor.java
+      ${CMAKE_CURRENT_SOURCE_DIR}/org/libjpegturbo/turbojpeg/TJDecompressor.java
+      ${CMAKE_CURRENT_SOURCE_DIR}/org/libjpegturbo/turbojpeg/TJTransformer.java)
+else()
+  add_custom_target(javah
+    COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJ
+    COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJCompressor
+    COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJDecompressor
+    COMMAND javah -d ${CMAKE_CURRENT_SOURCE_DIR} -classpath ${JAVACLASSPATH} org.libjpegturbo.turbojpeg.TJTransformer)
+endif()
 
 if(NOT DEFINED CMAKE_INSTALL_DEFAULT_JAVADIR)
   set(CMAKE_INSTALL_DEFAULT_JAVADIR "<CMAKE_INSTALL_DATAROOTDIR>/java")