Since commit v3.13.0~7^2 (FindBoost: Improve compiler prefix
detection for GCC 5+ and clang 4+, 2018-11-07) we run the internal
`_Boost_COMPILER_DUMPVERSION` macro on all UNIX platforms. Teach
the macro to tolerate missing `CMAKE_CXX_COMPILER_VERSION`, which
occurs when the CXX language is not enabled.
Fixes: #18624
Brad King