Building dev-libs/boost-1.76.0-r1 fails on my armv5tel with: ./boost/regex/v5/mem_block_cache.hpp:91:11: error: 'static_mutex' in namespace 'boost' does not name a type boost::static_mutex mut; ^~~~~~~~~~~~ This is a known bug https://github.com/boostorg/regex/issues/131 and there is a fix https://github.com/boostorg/regex/pull/132
We've got 1.77 in tree which will soon be stabled too.