link to static ffmpeg for gcc. old glibc compat option

lib names order is important. additional libs must be added.
clock_gettime in ffmpeg may be resolved as glibc2.17 symbol, while it's
only in librt on old OSes, like ubuntu 12.04. add -lrt can solve it.
define qtav version in project file only once. copy mkspecs for ifw

qt5 can use .qmake.conf in project root to store default configurations.
qt4 use .qmake.cache instead