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

- 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

Add Android HardFP support. Expose the "armeabi-v7a-hard with NEON"
ANDROID_ABI.

TEST= Add -DANDROID_NATIVE_API_LEVEL=21
-DANDROID_ABI="armeabi-v7a-hard with NEON"

to compile with hardfp. Checked the output of make verbose to
confirm -lm_hard is being linked. There are known test
failures but that can be fixed once this is in.