Master (15886b08aa5f05194633eba063d7412d0e4fd036) on Linux x86_64 (Focal codebase):
$ make -C depends HOST=x86_64-w64-mingw32
$ CONFIG_SITE=depends/x86_64-w64-mingw32/share/config.site ./configure
...
checking whether miniUPnPc API version is supported... yes
checking for boostlib >= 1.58.0 (105800) includes in "depends/x86_64-w64-mingw32/share/../include"... yes
checking for boostlib >= 1.58.0 (105800) lib path in "depends/x86_64-w64-mingw32/share/../lib/x86_64-mingw32"... no
checking for boostlib >= 1.58.0 (105800) lib path in "depends/x86_64-w64-mingw32/share/../lib64"... no
checking for boostlib >= 1.58.0 (105800) lib path in "depends/x86_64-w64-mingw32/share/../libx32"... no
checking for boostlib >= 1.58.0 (105800) lib path in "depends/x86_64-w64-mingw32/share/../lib"... yes
checking for boostlib >= 1.58.0 (105800)... yes
checking whether the Boost::System library is available... yes
configure: error: Could not find a version of the Boost::System library!