Do not build starpupy when CPUs are not available

Makefile.am

@@ -53,9 +53,11 @@ if STARPU_BUILD_STARPURM
 SUBDIRS += starpurm
 endif
 
+if STARPU_USE_CPU
 if STARPU_BUILD_STARPUPY
 SUBDIRS += starpupy
 endif
+endif
 
 if STARPU_BUILD_SC_HYPERVISOR
 SUBDIRS += sc_hypervisor