(gitignore) Add rules and comments
(cmake) Common Yasm arguments moved to a variable
(cmake) Eliminate redundant information in flags passed to compiler

CMAKE_C_FLAGS is a variable (empty by default) used by cmake to
configure command lines. Is is not necessary to set all _DEBUG,
_RELEASE, etc. variables to configure flags.

See http://www.cmake.org/Wiki/CMake_Useful_Variables#Compilers_and_Tools
for more information
(cmake) Minor fixes
(cmake) Less code to build hevc / hevc_sdl2