In case OPENCV_INCLUDE_INSTALL_PATH is absolute (i.e. starts with a "/"),
the path ends up with a double "/".

While this is mostly equivalent to a single slash, it may have a nasty
side effect when:
- OpenCV_INSTALL_PATH is empty
- OPENCV_INCLUDE_INSTALL_PATH is "/usr/include"
- the calling build script uses "-isystem" to specify the path to the
  headers of dependencies (to avoid warnings)

Specifying "-isystem /usr/include" breaks the path ordering, and GCC can
no longer find its "stdlib.h", thus CMake filters such statements.
Unfortunately it fails to do so when using "//usr/include".