6909026: Change GNU make version requirement to 3.81

Reviewed-by: robilad

README

@@ -26,6 +26,6 @@ Simple Build Instructions:
        gnumake all
      The resulting JDK image should be found in build/*/j2sdk-image
 
-where gnumake is GNU make 3.78.1 or newer, /usr/bin/make on Linux and
+where gnumake is GNU make 3.81 or newer, /usr/bin/make on Linux and
 /usr/sfw/bin/gmake or /opt/sfw/bin/gmake on Solaris.
 

README-builds.html

@@ -524,7 +524,7 @@
             A few notes about using GNU make:
             <ul>
                 <li>
-                    In general, you need GNU make version 3.78.1 or newer.
+                    In general, you need GNU make version 3.81 or newer.
                 </li>
                 <li>
                     Place the location of the GNU make binary in the <tt>PATH</tt>.