Remove everything after digits in SYSTEM

Instead of just removing periods and digits from SYSTEM, remove periods
and digits and everything after any digits. This handles unusual system
identifiers like powerpc-apple-darwin10.0.0d2.

I wasn't sure if periods can appear anywhere other than in the release
number so I retained the previous behavior of removing all periods
regardless where they are.

Makefile

@@ -31,7 +31,7 @@
 include config/version
 
 SHELL = /bin/sh
-SYSTEM ?= $(shell config/config.guess | cut -d - -f 3 | sed -e 's/[0-9\.]//g;')
+SYSTEM ?= $(shell config/config.guess | cut -d - -f 3 | sed -e 's/\.//g' -e 's/[0-9]\{1,\}.*//')
 SYSTEM.SUPPORTED = $(shell test -f config/Makefile.$(SYSTEM) && echo 1)
 
 ifeq ($(SYSTEM.SUPPORTED), 1)