nbody_CPU_SIMD.h: include stdlib.h for definition of size_t

Avoids this on Mac OS X:

	g++-mp-4.8  -DUSE_OPENMP -DHAVE_SSE -DNO_CUDA -O3 -march=native -ffast-math
		-fno-strict-aliasing -Wa,-q -fopenmp -pthread -I../chLib -c -o nbody_CPU_SSE.o
		nbody_CPU_SSE.cpp
	In file included from nbody.h:39:0,
					 from nbody_CPU_SSE.cpp:41:
	nbody_CPU_SIMD.h:47:5: error: 'size_t' has not been declared
		 size_t N
		 ^

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
GNUmakefile: on Darwin, use -Wa,-q for C++ compilation

This allows us to use AVX instructions, because GCC will invoke clang for
assembly.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
chThread_POSIX.h: drop include of <new>

It's unneeded and breaks on Mac for some reason.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
GNUmakefile: only add -Wa,-q on Mac if compiler is GCC

Signed-off-by: Steven Noonan <steven@uplinklabs.net>