java: support OpenJDK 10+

Starting OpenJDK 10, the 'javah' utility is no more provided [1], and
its functionality is provided by 'javac' itself.  Hence, do not error
out on missing 'javah', and store whether it was found; in case it is
not, then:
1) assume 'javac' has the -h parameter to generate the C header, and
   make use of it
2) tell the buildsystem that com_redhat_et_libguestfs_GuestFS.h depends
   on the JAR, since the the header is generated at the step (1)

[1] https://bugs.java.com/view_bug.do?bug_id=JDK-8182758

java/Makefile.am

@@ -77,6 +77,9 @@ if HAVE_JAVA
 
 JAVAC_FLAGS = $(EXTRA_JAVAC_FLAGS) -encoding utf-8
 JAVADOC_FLAGS = -encoding utf-8
+if !HAVE_JAVAH
+JAVAC_FLAGS += -h .
+endif
 
 # Java source.
 
@@ -125,10 +128,18 @@ libguestfs_jni_la_LDFLAGS = -version-info $(JNI_VERSION_INFO) -shared
 
 BUILT_SOURCES = com_redhat_et_libguestfs_GuestFS.h
 
+if HAVE_JAVAH
+
 com_redhat_et_libguestfs_GuestFS.h: $(java_prefix)/GuestFS.class
 	rm -f $@
 	$(JAVAH) -classpath $(srcdir):. com.redhat.et.libguestfs.GuestFS
 
+else
+
+com_redhat_et_libguestfs_GuestFS.h: libguestfs-${VERSION}.jar
+
+endif
+
 # Documentation.
 
 noinst_SCRIPTS = doc-stamp

m4/guestfs-java.m4

@@ -73,9 +73,7 @@ if test "x$with_java" != "xno"; then
         else
             JAVAC="$JAVA/bin/javac"
         fi
-        if test ! -x "$JAVA/bin/javah"; then
-            AC_MSG_ERROR([missing $JAVA/bin/javah binary])
-        else
+        if test -x "$JAVA/bin/javah"; then
             JAVAH="$JAVA/bin/javah"
         fi
         if test ! -x "$JAVA/bin/javadoc"; then
@@ -165,4 +163,5 @@ if test "x$with_java" != "xno"; then
     AC_SUBST(JNI_VERSION_INFO)
 fi
 
+AM_CONDITIONAL([HAVE_JAVAH],[test -n "$JAVAH"])
 AM_CONDITIONAL([HAVE_JAVA],[test "x$with_java" != "xno" && test -n "$JAVAC"])