update instructions for g++ 6

Author: Daniel Kroening

COMPILING

@@ -44,12 +44,18 @@ We assume that you have a Debian/Ubuntu or Red Hat-like distribution.
 
    git clone https://github.com/diffblue/cbmc cbmc-git
 
-2) Do
+2) On Debian/Ubuntu, do
 
    cd cbmc-git/src
    make minisat2-download
    make
 
+   On Redhat/Fedora etc., do
+
+   cd cbmc-git/src
+   make minisat2-download
+   make CXX=g++-6
+
 
 COMPILATION ON SOLARIS 11
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