AutoBuffer::allocate(nsz) didn't work properly when
(sz < nsz < fixed_size). In this case sz remained unchanged.

Added missing SimpleBlobDetector parameters to features2d_init.cpp.

Some CMake 3.x (2.8.12.2 is fine) provides broken build configurations
with following build error message:
    ld: fatal error: -soname: must take a non-empty argument

Linker command line sample contains this:
"-shared -Wl,-soname, -o"

The number of gaussians involved in a mixture is supposed
to be dynamically adjusted. After being increased, the number
of gaussians can't be reduced anymore.

It seems to be a regression as the legacy code
located in modules/legacy/src/bgfg_gaussmix.cpp allows to reduce
such number of gaussians.

Command line generation routine uses "-isystem" to include headers outside of "<opencv>/modules" folder, but GCC 6 does not work when passed "-isystem /usr/include" option.