Document steps to perform CMake configuration without network access

Build environments may be configured in a way that disallows network
access. The make-based build system already supported this as
downloading and building solvers is a separate step. For CMake, document
the procedure to facilitate this.

Fixes: #6582

Author: tautschnig

COMPILING.md

@@ -82,6 +82,16 @@ files.
    to generate IDE projects by supplying the `-G` flag.  Run `cmake -G` for a
    comprehensive list of supported back-ends.
 
+   As part of this step, CMake will download the back-end solver (see Section
+   "Compiling with alternative SAT solvers" in this document for configuration
+   options). Should it be necessary to perform this step without network access,
+   a solver can be downloaded ahead of the above `cmake` invocation as follows:
+   ```
+   mkdir -p build/minisat2-download/minisat2-download-prefix/src/
+   wget http://ftp.debian.org/debian/pool/main/m/minisat2/minisat2_2.2.1.orig.tar.gz \
+     -O build/minisat2-download/minisat2-download-prefix/src/minisat2_2.2.1.orig.tar.gz
+   ```
+
    On macOS >10.14, the build will fail unless you explicitly specify
    the full path to the compiler. This issue is being tracked
    [here](https://github.com/diffblue/cbmc/issues/4956). The invocation thus