Useful for debug purposes:
    cmake -DDEBUG_POST="ocv_cmake_dump_vars(CUDA)" .
    cmake -DCMAKE_PRE="ocv_cmake_dump_vars(\"OPENCL|opencl\")" .

PCL PR 1205

This reverts commit f45da986.

  * fix issue 6536
  * increase threshold in particular tests due to the difference of fmsub and fsub

fix compiler error when building with gnu-5 toolchain (eg as currently shipped with crystax.org android ndk, and presumably soon with android studio).

"ts_perf.cpp ....error: 'errno' was not declared in this scope
      int err=errno;"

Commit:
  6d54abbd

Allow to use custom implementations of CV_XADD (passed via -D of CXXFLAGS)