+make version minimum if GNU make

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@39607 15284696-431f-4ddb-bdfa-cd5b030d7da7

REQUIREMENTS.html

@@ -28,7 +28,7 @@ to install some of them.)
 <br>gcc: <a href="http://www.gnu.org/software/gcc/gcc.html">http://www.gnu.org/software/gcc/gcc.html</a>
 <br>
 
-<li><B>GNU make</B> is recommended
+<li><B>GNU make</B> is recommended (at least version 3.81)
 <br><a href="http://www.gnu.org/software/make/make.html">http://www.gnu.org/software/make/make.html</a>
 <br>