java: support OpenJDK 10+

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

Gbp-Pq: Name 0013-java-support-OpenJDK-10.patch