Emphasize that groff must use -j1

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@11275 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter06/groff.xml

@@ -53,9 +53,9 @@
 
 <screen><userinput remap="configure">PAGE=<replaceable>&lt;paper_size&gt;</replaceable> ./configure --prefix=/usr</userinput></screen>
 
-    <para>Compile the package:</para>
+    <para>This package does not support parallel build. Compile the package:</para>
 
-<screen><userinput remap="make">make</userinput></screen>
+<screen><userinput remap="make">make -j1</userinput></screen>
 
     <para>This package does not come with a test suite.</para>