Add instructions on how to link against IPASIR solvers through CMAKE.

Add more instructions on linking against custom IPASIR
supporting solvers through CMake.

Author: NlightNFotis

CMakeLists.txt

@@ -81,7 +81,7 @@ set(enable_cbmc_tests on CACHE BOOL "Whether CBMC tests should be enabled")
 
 set(sat_impl "minisat2" CACHE STRING
     "This setting controls the SAT library which is used. Valid values are
-    'minisat2', 'glucose', 'cadical' or 'ipasir-cadical'"
+    'minisat2', 'glucose', 'cadical', 'ipasir-cadical' or 'ipasir-custom'"
 )
 
 if(${enable_cbmc_tests})

COMPILING.md

@@ -467,10 +467,22 @@ Note that at the time of writing this has been tested to work with the CaDiCaL
 It's also possible to build CBMC using CaDiCaL through IPASIR via `cmake`,
 controlled with the flag `-Dsat_impl=ipasir-cadical`, like so:
 
-```
+```sh
 $ cmake -Bbuild_ipasir -S. -Dsat_impl=ipasir-cadical
 ```
 
+An advanced user may also take a more adventurous route, trying to link
+CBMC against any solver supporthing the IPASIR interface. To do that,
+an invocation like this is needed:
+
+```sh
+$ cmake -Bbuild_ipasir -S . -Dsat_impl=ipasir-custom -DIPASIR=<source_location> -DIPASIR_LIB=<lib_location>
+```
+
+with `<source_location>` being the absolute path to the folder containing
+the solver implementation and `<lib_location>` being the absolute path that
+contains a precompiled static library of the solver (`.a` file).
+
 #### Compiling with Riss via IPASIR
 
 The [Riss](https://github.com/conp-solutions/riss) solver supports the