set GA_Update flag in order remove ERROR 8: band 1: Attempt to write to read only dataset inGDALRasterBand::Fill()

https://github.com/Itseez/opencv/issues/4791

Update docs according to PR #5127

- IPP is disabled by default when compiler is mingw (couldn't make it
work)
- fixed some warnings
- fixed some `__GNUC__` version checks (for correctness and convenience)
- removed UTF-8 BOM from hough.cpp (fixes #5253)

Fixed small mistake in documentation.
The prism coefficients in y'' equation were mistakenly written as s1,s2 instead of s3,s4

The URL http://www.nptel.iitm.ac.in/courses/106108057/26 under Additional Resource section gives a 404. I've added the updated one i.e. http://www.nptel.ac.in/courses/106108057/26