Add a --disable-docs to configure to allow doc build to be disabled

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -177,7 +177,7 @@ softmmu="yes"
 linux_user="no"
 darwin_user="no"
 bsd_user="no"
-build_docs="no"
+build_docs="yes"
 uname_release=""
 curses="yes"
 pthread="yes"
@@ -492,6 +492,8 @@ for opt do
   ;;
   --with-pkgversion=*) pkgversion=" ($optarg)"
   ;;
+  --disable-docs) build_docs="no"
+  ;;
   *) echo "ERROR: unknown option $opt"; show_help="yes"
   ;;
   esac
@@ -1236,9 +1238,8 @@ EOF
 fi
 
 # Check if tools are available to build documentation.
-if [ -x "`which texi2html 2>/dev/null`" ] && \
-   [ -x "`which pod2man 2>/dev/null`" ]; then
-  build_docs="yes"
+if test "$build_docs" = "yes" -a \( ! -x "`which texi2html 2>/dev/null`" -o ! -x "`which pod2man 2>/dev/null`" \) ; then
+  build_docs="no"
 fi
 
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