update checksum for cfitsio
add note about winpthreads version
update package qt5
update package gtk2
update package openscenegraph
doc update: minimum supported Debian version
add package sfml
package sfml: use pkg-config and fix whitespace
package sfml: fix line endings and convert patches to git format
package vtk6: set CMAKE_REQUIRE_LARGE_FILE_SUPPORT
doc update: add note about date portability variable
doc update: add note about patch line endings and .gitattributes
doc update: add note about openssl sha1 to calculate checksum
doc update: add notes about supported toolchains and MXE_TARGETS variable