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
Merge pull request #71 from dequis/linux-natives-build-script

Various improvements to the linux natives build script