bootstrap: support a prereq of 'rpcgen -' on RHEL5

On RHEL 5, 'rpcgen --version' spews usage to stderr with status 1,
rather than a version string to stdout with status 0.  But libvirt
merely requires a prereq of 'rpcgen -' - that is, it must exist,
but need not have a well-behaved --version.

* build-aux/bootstrap (check_versions): When no specific version
is required, merely check that the app produces an exit status
that indicates its existence.

Signed-off-by: Eric Blake <eblake@redhat.com>

ChangeLog

@@ -1,5 +1,10 @@
 2011-05-11  Eric Blake  <eblake@redhat.com>
 
+	bootstrap: support a prereq of 'rpcgen -' on RHEL5
+	* build-aux/bootstrap (check_versions): When no specific version
+	is required, merely check that the app produces an exit status
+	that indicates its existence.
+
 	maint.mk: drop redundant check
 	* top/maint.mk (sc_the_the): Delete; sc_prohibit_doubled_word does
 	the same but better.

build-aux/bootstrap

@@ -1,6 +1,6 @@
 #! /bin/sh
 # Print a version string.
-scriptversion=2011-05-03.08; # UTC
+scriptversion=2011-05-11.17; # UTC
 
 # Bootstrap this package from checked-out sources.
 
@@ -425,11 +425,21 @@ check_versions() {
         GZIP) ;; # Do not use $GZIP:  it contains gzip options.
         *) eval "app=\${$appvar-$app}" ;;
     esac
+    if [ "$req_ver" = "-" ]; then
+      # Merely require app to exist; not all prereq apps are well-behaved
+      # so we have to rely on $? rather than get_version.
+      $app --version >/dev/null 2>&1
+      if [ 126 -le $? ]; then
+        echo "$me: Error: '$app' not found" >&2
+        ret=1
+      fi
+    else
+      # Require app to produce a new enough version string.
       inst_ver=$(get_version $app)
       if [ ! "$inst_ver" ]; then
         echo "$me: Error: '$app' not found" >&2
         ret=1
-    elif [ ! "$req_ver" = "-" ]; then
+      else
         latest_ver=$(sort_ver $req_ver $inst_ver | cut -d' ' -f2)
         if [ ! "$latest_ver" = "$inst_ver" ]; then
           echo "$me: Error: '$app' version == $inst_ver is too old" >&2
@@ -437,6 +447,7 @@ check_versions() {
           ret=1
         fi
       fi
+    fi
   done
 
   return $ret