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)

Add information about python bindings

Fix findContours crash for very large images (v2.4)

* Cast step to size_t in order to avoid integer overflow when processing very large images

* Change assert to CV_Assert

'const' is available in OpenCV 3.x only