Various improvements to the linux natives build script

- Detect if the JDK path is correct by checking include/jni.h instead
  (prevents false positives if "/usr" is incorrectly detected as the path)
- Detect opencv and tesseract using pkg-config
- Update DEVJAVA and DEVNATIVE paths to reflect current repo layout
- Drop DEVLIBS path variable - no direct equivalent in this repo layout.
  Output libVisionProxy.so to current directory instead
- Don't run runBuildVisionProxy.sh if SDK detection failed
- Use set -e to abort runBuildVisionProxy.sh on the first error

-------

How I got here: The bundled .so files compile against opencv 2.3. The supplemental linux zip linked from the quickstart docs doesn't exist, compiling the 1.0.1 version results in missing symbols. Also I think the installer doesn't pick up the stuff i put in libs/, or it fails with an error that doesn't output anything interesting. I'm just avoiding that and copying the generated .so inside the jar to `META-INF/libs/linux/libs64/libVisionProxy.so`

The way the 'documentation' is split between old website/new website/launchpad/github/readthedocs/etc is very confusing, btw. But whatever, it works now.