configure: Clarify error message about missing dependencies

Took me a moment to figure out that the appropriate response to
"need program file" was to install the program named "file", not
to think "If I didn't need the program file, why would I be
compiling things?".

configure

@@ -33,8 +33,8 @@ need_ok() {
 
 need_cmd() {
     if command -v $1 >/dev/null 2>&1
-    then msg "found program $1"
-    else err "need program $1"
+    then msg "found program '$1'"
+    else err "program '$1' is missing, please install it"
     fi
 }