minor updates to README

README.txt

@@ -20,13 +20,14 @@ machines.
     ./prepare.sh
 
 2- Configure codes-base. This can be done in the source directory or in a
-   dedicated build directory if you prefer out-of-tree builds.
+   dedicated build directory if you prefer out-of-tree builds.  The CC
+   environment variable must refer to an MPI compiler.
 
-    ./configure --with-ross=/path/to/ross/install
+    ./configure --with-ross=/path/to/ross/install --prefix=/path/to/codes-base/install CC=mpicc
 
 3 - Build codes-base
 
-    make clean && make
+    make clean && make && make install
 
 4 - codes-base uses flex and bison (or lex and yacc) to generate several parsers.
 These tools auto-generate C source files. To remove all of the autogenerated