Introduce a configure option to disable building of qemu documentation.
Gentoo Bug #192427

Index: xen-3.2.1/tools/ioemu/configure
===================================================================
--- xen-3.2.1.orig/tools/ioemu/configure
+++ xen-3.2.1/tools/ioemu/configure
@@ -214,6 +214,8 @@ for opt do
   ;;
   --disable-sdl) sdl="no"
   ;;
+  --enable-docs) build_docs="yes"
+  ;;
   --enable-coreaudio) coreaudio="yes"
   ;;
   --enable-alsa) alsa="yes"
@@ -295,6 +297,7 @@ echo "  --enable-coreaudio       enable 
 echo "  --enable-alsa            enable ALSA audio driver"
 echo "  --enable-fmod            enable FMOD audio driver"
 echo "  --enabled-dsound         enable DirectSound audio driver"
+echo "  --enable-docs            enable building of docs (requires texi2html and pod2man)"
 echo "  --disable-vnc-tls        disable TLS encryption for VNC server"
 echo "  --enable-system          enable all system emulation targets"
 echo "  --disable-system         disable all system emulation targets"
@@ -576,8 +579,9 @@ EOF
 fi
 
 # Check if tools are available to build documentation.
-if [ -x "`which texi2html`" ] && [ -x "`which pod2man`" ]; then
-  build_docs="yes"
+if [ ! -x "`which texi2html`" ] || [ ! -x "`which pod2man`" ]; then
+  echo "Missing texi2html or pod2man, will not build docs"
+  build_docs="no"
 fi
 
 if test "$mingw32" = "yes" ; then