build: fix configure compatiblity with POSIX shells

The kernel version detection code uses some bashisms, which makes the
build fail on Debian systems where /bin/sh links to dash. Replace with
POSIX-conforming commands at the cost of requiring awk.

configure.ac

@@ -44,21 +44,15 @@ regular_CFLAGS="-Wall -Waggregate-return -Wmissing-declarations \
 
 if test -n "$kbuilddir"; then
 	AC_MSG_CHECKING([kernel version that we will build against])
-	krel="$(make -sC "$kbuilddir" M=$PWD kernelrelease)";
-	kmajor="${krel%%[[^0-9]]*}";
-	kmajor="$(($kmajor+0))";
-	krel="${krel:${#kmajor}}";
-	krel="${krel#.}";
-	kminor="${krel%%[[^0-9]]*}";
-	kminor="$(($kminor+0))";
-	krel="${krel:${#kminor}}";
-	krel="${krel#.}";
-	kmicro="${krel%%[[^0-9]]*}";
-	kmicro="$(($kmicro+0))";
-	krel="${krel:${#kmicro}}";
-	krel="${krel#.}";
-	kstable="${krel%%[[^0-9]]*}";
-	kstable="$(($kstable+0))";
+	krel="$(make -sC "$kbuilddir" M=$PWD kernelrelease | $AWK -v 'FS=[[^0-9.]]' '{print $1; exit}')"
+	save_IFS="$IFS"
+	IFS='.'
+	set x $krel
+	IFS="$save_IFS"
+	kmajor="$(($2+0))"
+	kminor="$(($3+0))"
+	kmicro="$(($4+0))"
+	kstable="$(($5+0))"
 	if test -z "$kmajor" -o -z "$kminor" -o -z "$kmicro"; then
 		echo "WARNING: Version detection did not succeed. Continue at own luck.";
 	else