stir_shaken: Build with OpenSSL on Ubuntu 20.04

This fixes the unit tests CI on Ubuntu 20.04, where we don't want to
build against WolfSSL (no OID support yet).

Author: liviuchircu

.github/workflows/unittests.yml

@@ -40,6 +40,8 @@ jobs:
     env:
       COMPILER: ${{ matrix.compiler }}
       BUILD_OS: ubuntu:${{ matrix.os }}
+      # Focal's wolfSSL lacks the custom-OID APIs needed for TNAuthList.
+      STIR_SHAKEN_OPENSSL: ${{ matrix.os == '20.04' && '1' || '' }}
 
     strategy:
       fail-fast: false

modules/stir_shaken/README.md

@@ -32,7 +32,16 @@ The following libraries or applications must be installed before running
 OpenSIPS with this module loaded:
 
 
-- *wolfssl (libwolfssl)*.
+- *wolfssl (libwolfssl)*, or *OpenSSL (libcrypto)* when building with
+  `STIR_SHAKEN_OPENSSL=1`.
+
+Older wolfSSL releases, such as Ubuntu 20.04's packaged version, cannot handle
+the custom TNAuthList OID required by this module. Use the OpenSSL backend on
+these systems:
+
+```console
+make -B -j4 modules/stir_shaken STIR_SHAKEN_OPENSSL=1
+```
 
 
 ### Exported Parameters