Clarify the logic that the build system uses for setting the default solver

Author: zhassan-aws

COMPILING.md

@@ -392,8 +392,12 @@ For example, to build CBMC with MiniSAT2 and Glucose, do:
 make -C src minisat2-download glucose-download
 make -C src MINISAT2=../../minisat-2.2.1 GLUCOSE=../../glucose-syrup
 ```
-In that case, MiniSAT2 will be used by default. To switch to glucose at runtime,
-run CBMC with `--sat-solver glucose`.
+The build sets the default solver based on the priority defined by the
+`#if/#elif` tree defined at the end of
+[`src/solvers/sat/satcheck.h`](https://github.com/diffblue/cbmc/blob/develop/src/solvers/sat/satcheck.h).
+In the above example, MiniSAT2 will be set as the default solver because it is
+listed before Glucose. To switch to glucose at runtime, run CBMC with
+`--sat-solver glucose`.
 
 For CMake the alternatives can be built with the following arguments to `cmake`
 for glucose `-Dsat_impl=glucose`.