Use "expr substr" instead of "substr" which appears to be more portable.

Fixes build problem on Blue Waters.

configure

@@ -36,7 +36,7 @@ fi
 
 # `git archive` (man gitattributes(5)) fills this in, otherwise we do it manually...
 COMMIT='$Format:%H$'
-if [ "$(substr "$COMMIT" 1 1)" = '$' ]; then
+if [ "$(expr substr "$COMMIT" 1 1)" = '$' ]; then
 	if [ "$GIT" = y ]; then
 		COMMIT="$(git rev-parse HEAD)"
 	else
@@ -54,7 +54,7 @@ fi
 
 # The date is the commit date of the commit, or now if the working tree is dirty.
 DATE='$Format:%ci$'
-if [ "$(substr "$DATE" 1 1)" = '$' ]; then
+if [ "$(expr substr "$DATE" 1 1)" = '$' ]; then
 	if [ "$GIT" = y ]; then
 		DATE="$(git log -1 --pretty=format:%ci HEAD)"
 	else
@@ -72,7 +72,7 @@ elif [ -n "$CCTOOLS_BRANCH" ]; then
 	BRANCH="$CCTOOLS_BRANCH"
 else
 	BRANCH='$Format:%d$'
-	if [ "$(substr "$BRANCH" 1 1)" = '$' ]; then
+	if [ "$(expr substr "$BRANCH" 1 1)" = '$' ]; then
 		if [ "$GIT" = y ]; then
 			BRANCH="$(git rev-parse --abbrev-ref HEAD)"
 		else
@@ -95,7 +95,7 @@ else
 	fi
 fi
 
-SOURCE="${BRANCH}:$(substr "$COMMIT" 1 8)${DIRTY}"
+SOURCE="${BRANCH}:$(expr substr "$COMMIT" 1 8)${DIRTY}"
 VERSION="$MAJOR.$MINOR.$MICRO [${SOURCE}]"
 
 # Bourne shell is tricky when you use backtick command substitution backslash