spice_common.h provides an ASSERT macro, no need to duplicate it
in many places. For now client/debug.h keeps its own copy since
debug.h and spice_common.h have clashes on other macros which are
trickier to unify.
When using config.h, it must be the very first include in all source
files since it contains #define that may change the compilation process
(eg libc structure layout changes when it's used to enable large file
support on 32 bit x86 archs). This commit adds it at the beginning
of all .c and .cpp files
