diff --git a/configure b/configure
index 85e4eea..45d4fb6 100644
--- a/configure
+++ b/configure
@@ -1915,7 +1915,10 @@ git submodule update --init --remote --recursive
 # archive into src/<whatever>.
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: initializing vendors..." >&5
 printf "%s\n" "$as_me: initializing vendors..." >&6;}
-sh -xc "cd vendor/html-theme-ref && sh ./configure"
+
+# prefix for vendor subcommand output
+sub_prefix="vendor/html-theme-ref: "
+sh -c "cd vendor/html-theme-ref && sh ./configure | sed 's|^|$sub_prefix|'"
 
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: initializing python3 venv..." >&5
 printf "%s\n" "$as_me: initializing python3 venv..." >&6;}
diff --git a/configure.ac b/configure.ac
index d6c9157..02bee02 100644
--- a/configure.ac
+++ b/configure.ac
@@ -14,7 +14,10 @@ git submodule update --init --remote --recursive
 # by storing the build package archive under vendor/ instead and untar the
 # archive into src/<whatever>.
 AC_MSG_NOTICE([initializing vendors...])
-sh -xc "cd vendor/html-theme-ref && sh ./configure"
+
+# prefix for vendor subcommand output
+sub_prefix="vendor/html-theme-ref: "
+sh -c "cd vendor/html-theme-ref && sh ./configure | sed 's|^|$sub_prefix|'"
 
 AC_MSG_NOTICE([initializing python3 venv...])
 python3 -m venv .venv