Document CMake version requirement and fail if it's not satisfied

Author: NlightNFotis

CMakeLists.txt

@@ -259,7 +259,7 @@ endif()
 option(WITH_RUST_API "Build with the CPROVER Rust API" OFF)
 if(${CMAKE_VERSION} VERSION_LESS "3.19.0" AND WITH_RUST_API)
     message("Unable to build the Rust API without version CMake 3.19.0")
-    message("(The Rust build depends on CMake plugins with dependencies on CMake 3.19.0 and above)")
+    message(FATAL_ERROR "(The Rust build depends on CMake plugins with dependencies on CMake 3.19.0 and above)")
 else()
     if(WITH_RUST_API)
     include(FetchContent)

COMPILING.md

@@ -570,6 +570,8 @@ CBMC, you need two things:
 
 - Rust/Cargo, instructions the installation of which can be found [here](https://www.rust-lang.org/tools/install), and
 - CMake (the Rust API doesn't support being built with `Make` yet)
+  - Version `3.19.0`+ is required for the build, because of a dependency on CMake modules
+    that need the higher version themselves.
 
 Provided these two are available, you can perform a CBMC build *including*
 the Rust API by passing in the option `WITH_RUST_API` to `CMake` like this: