Improve FreeBSD support

[michael-o]

• Detect JAVA_HOME automatically as advised by man javavm if not set
• Avoid a compile-time error on FreeBSD 10.3-RELEASE along with clang if the
  OS-specific include directory is not passed to the preprocessor, BSD/OS is dead for 15 years+ and does not correspond to modern BSDs like FreeBSD or OpenBSD. See config.sub.

Both compiles now perfectly on FreeBSD 9 and 10 in 32 bit and 64 bit, Java 7 and Java 8.

Error with clang (FreeBSD clang version 3.4.1 (tags/RELEASE_34/dot1-final 208032) 20140512, Target: x86_64-unknown-freebsd10.3, Thread model: posix):

configure:11640: JAVA_HOME was set, checking to see if it's a JDK we can use...
configure:11645: checking if '/usr/local/openjdk8' is a JDK
configure:11706: cc -c -g -O2  -I/usr/local/openjdk8/include conftest.c >&5
In file included from conftest.c:21:
/usr/local/openjdk8/include/jni.h:45:10: fatal error: 'jni_md.h' file not found
#include "jni_md.h"
         ^
1 error generated.
configure:11706: $? = 1

Working library:

$ file ./target/native-build/target/lib/libjansi-1.7-SNAPSHOT.so
./target/native-build/target/lib/libjansi-1.7-SNAPSHOT.so: ELF 64-bit LSB shared object, x86-64, version 1 (FreeBSD), dynamically linked, not stripped

[michael-o]

@chirino Can you have a look please? I'd like to upstream everyhting into Jansi associatd with FreeBSD.

[chirino]

Thanks!

[michael-o]

@chirino Thank you. Looking forward to the next release to solve fusesource/jansi-native#5.

[chirino]

Just cut 1.14