contrib: improve the detection of SVN

extras/contrib/bootstrap

@@ -66,12 +66,12 @@ else
 	exit 1
 fi
 
-if which svn >/dev/null; then
-	echo "SVN = svn" >> config.mak
-elif which /sw/bin/svn >/dev/null; then
+if test -f /sw/bin/svn; then
         echo "SVN = /sw/bin/svn" >> config.mak
+elif svn help >/dev/null 2>&1; then
+        echo "SVN = svn" >> config.mak
 else
-	echo "You don't have a subversion client installed." >&2
+        echo "You do not have a subversion client installed." >&2
 fi
 
 echo "PREFIX = `pwd`" >> config.mak