Switch from 'which' to 'command -v' to increase portability (#185)

In minimal buildroots 'which' might not be installed, while
'command -v' is a shell built-in in all POSIX 2008 compliant
shells, supported by at least ash, bash, busybox, dash, ksh,
mksh and zsh.

configure

@@ -832,7 +832,7 @@ fi
 
 if [ -z "$PYTHON" ]; then
 	for pp in python python3 python2; do
-		if which $pp > /dev/null; then
+		if command -v $pp > /dev/null; then
 			PYTHON=$pp
 			break
 		fi