configure: prefer posix threads if available

1) it's faster since SDL_Threads are a wrapper around it
   (2 levels of indirection instead one)
2) posix threads are more powerful, and have the ability to
   set the required stack size, unlike SDL.
build script: remove make clean

1) it destroys the results from the previous configure run
2) because of that, configure is run a second time,
3) wasting our time, and
4) throwing away all our settings.
a2091: depends on uae_msleep
memory.c: supply nop impl of protect_roms

this function is used by akiko.c when cd32 is enabled.
savestate: save_dmac was renamed
scsiemul.c: supply missing variable and use semaphore correctly

using the posix thread impl, uae_sem_t is a struct, not an int.
do not read /proc/cpuinfo over and over again
gtk ui: react on button clicks

previously gui events weren't processed at all.
the buttons STOP and START remain disfunctional, as the code used
by them has been removed.
optimally these buttons should be removed as well.