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
clang static analyzer warned that 'len' was computed but never
used in glc_vertex2d. glc_stroke_line_dash has side effects so
we have to call it, but we don't need to save its return value
since it's not used.
