Added support of `--disable-opencv` option.

* bootstrap: Added the support of `--disable-opencv` command line option.
* jamroot:
  * Added the support of `--disable-opencv` command line option.
  * Only the last one of `--enable-opencv` and `--disable-opencv` command
    line options is effective.
Merge pull request #117 from Cryolite/feature/disableOpencv

Added support of `--disable-opencv` option.