javacomp-script, javacomp: Fix support for Java 7, 8, 9.

javacomp-script, javacomp: Fix support for Java 7, 8, 9.

* lib/javaversion.h: Update comments.
* lib/javacomp.h: Likewise.
* lib/javacomp.c (default_target_version, source_version_index,
get_goodcode_snippet, get_failcode_snippet): Recognize "9" instead of
"1.9".
(TARGET_VERSION_BOUND): Bump to 9.
(target_version_index, corresponding_classfile_version): Recognize "9"
instead of "1.9".
(get_source_version_for_javac): New function.
(is_envjavac_nongcj_usable, is_javac_usable): Add
source_version_for_javac argument.
(compile_java_class): Determine and pass source_version_for_javac.
* m4/javacomp.m4: Recognize version '9' instead of '1.9'. When invoking
$JAVAC or javac, pass '-source 1.6' instead of '-source 1.5' when
appropriate.