Change CPPFLAGS to -D${MACHINE}=${MACHINE} instead of just -D${MACHINE}

This makes it possible to define header files on the command line that
might include ${MACHINE} somewhere in the path.  This might be used in
evbppc, for example, when defining PPC_PCI_MACHDEP_IMPL as, for example:

PPC_PCI_MACHDEP_IMPL="<arch/evbppc/sandpoint/pci_machdep.h>"

which will be included as

#include PPC_PCI_MACHDEP_IMPL

Prior to this change, the compile would fail trying to include
<arch/evbppc/1/pci_machdep.h>

sys/arch/powerpc/conf/Makefile.powerpc

@@ -1,4 +1,4 @@
-#	$NetBSD: Makefile.powerpc,v 1.28 2003/09/29 05:14:23 simonb Exp $
+#	$NetBSD: Makefile.powerpc,v 1.29 2004/01/06 18:44:57 briggs Exp $
 #
 # Makefile for NetBSD
 #
@@ -37,7 +37,7 @@ GENASSYM=	${POWERPC}/${PPCDIR}/genassym.cf
 ##
 ## (2) compile settings
 ##
-CPPFLAGS+=	-D${MACHINE}
+CPPFLAGS+=	-D${MACHINE}=${MACHINE}
 CWARNFLAGS+=	-Wreturn-type
 CFLAGS+=	-msoft-float ${CCPUOPTS}
 AFLAGS+=	${AOPTS}