Changes for 3b1 suggested by Andy Fyfe <andy@scp.caltech.edu>

Changes for 3b1 suggested by Andy Fyfe <andy@scp.caltech.edu>
* 7300.h [__GNUC__]: #define HAVE_ALLOCA.
[not __GNUC__]: #define SWITCH_ENUM_BUG, C_ALLOCA, and STACK_DIRECTION.
(HAVE_SYSVIPC, USE_UTIME): #define these.
(memmove): We don't have this; call safe_bcopy.
(CRT0_COMPILE): #define this, so we always use the system's cc.
* config.h.in: Add #undefs for HAVE_MKDIR and HAVE_RMDIR, for
configure to edit.
* sysdep.c [not HAVE_MKDIR] (mkdir): New function, taken from tar,
for use on systems lacking the mkdir function.
[not HAVE_RMDIR] (rmdir): New function, taken from tar, for use on
systems lacking the rmdir function.
* ymakefile (crt0.o): Replace the compiler and switches with the
symbol CRT0_COMPILE.
(CRT0_COMPILE): If the system and machine description files have
left this undefined, then define it to do the same as the old
crt0.o rule used to.