look for "jni.h" in gcj-jdk path, needed to find "jni.h" with gcj-jdk_4.9.0

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>
(cherry picked from commit 0f4120c0115e7977ae7c03458addcc2b2916db07)
pass '-classpath' option (gcj/javah ignores CLASSPATH environment variable).

 This should not affect OpenJDK which understands '-classpath' as well.

 With gcj-jdk we still get FTBFS later:

 ~~~~
    java/native/libcephfs_jni.cc:2878:55: error: invalid conversion from 'const jbyte* {aka const signed char*}' to 'jbyte* {aka signed char*}' [-fpermissive]
                 reinterpret_cast<const jbyte*>(rawAddress));
                                                           ^
    In file included from java/native/libcephfs_jni.cc:27:0:
    /usr/lib/gcc/x86_64-linux-gnu/4.8/include/jni.h:1471:8: error:   initializing argument 4 of 'void _Jv_JNIEnv::SetByteArrayRegion(jbyteArray, jsize, jsize, jbyte*)' [-fpermissive]
       void SetByteArrayRegion (jbyteArray val0, jsize val1, jsize val2, jbyte * val3)
            ^
    make[5] *** [java/native/libcephfs_jni_la-libcephfs_jni.lo] Error 1
 ~~~~

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>
(cherry picked from commit 89fe0353582bde7e2fbf32f1626d430a20002dd0)
prioritise use of `javac` executable (gcj provides it through alternatives).

 On Debian this fixes FTBFS when gcj-jdk and openjdk-7-jdk are installed at
 the same time because build system will use default `javac` executable
 provided by current JDK through `update-alternatives` instead of blindly
 calling GCJ when it is present.

Signed-off-by: Dmitry Smirnov <onlyjob@member.fsf.org>
(cherry picked from commit 8b682d167e4535df582f1c77542e2b1ea0981228)