allow config to override kernel version

INSTALL

@@ -3,7 +3,25 @@
 PLAN9=`pwd` export PLAN9
 PATH=$PLAN9/bin:$PATH export PATH
 
+echo "Resetting $PLAN9/config"
+rm -f $PLAN9/config
+
 (
+if [ `uname` = Linux ]; then
+	# On Linux, we use the kernel version to decide whether
+	# to use pthreads or not.  On 2.6 versions that aren't
+	# linking with NPTL by default, pretend to be an older kernel.
+	echo "Running on Linux: checking for NPTL..."
+	gcc lib/linux-isnptl.c
+	if ./a.out
+	then	
+		echo "	NPTL found."
+		echo "SYSVERSION=2.6" >$PLAN9/config
+	else
+		echo "	NPTL not found."
+		echo "SYSVERSION=2.4" >$PLAN9/config
+	fi
+fi
 echo "Building mk..."
 cd src
 make