poco 1.12.0 has switched from PCRE 1 to PCRE 2, and ships FindPCRE2.cmake rather than FindPCRE.cmake. However, Foundation/cmake/PocoFoundationConfig.cmake still includes:

find_dependency(PCRE REQUIRED)

This causes mumble (which depends on poco) to fail to configure, because CMake can't find FindPCRE.cmake. Changing PCRE to PCRE2 here appears to fix it.