Without this change, the directory of `javadoc` command must be
included in the `PATH` environment variable at `make`-time.
Paths of `javac`, `javah`, and `jar` commands are detected in
`configure`. So the path of `javadoc` also should be detected.
Signed-off-by: KAWASHIMA Takahiro <t-kawashima@jp.fujitsu.com>
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