Eu não uso pra não ter que fazer isso, portanto não tem como eu testar aqui.
Mas tem instruções:
d:\CVSFiles\vszakats\contrib\hbssl>hbmk2 hbssl
hbmk2: Processing environment options: -comp=msvc
hbmk2: Building sub-project (level 2): hbssls.hbp
hbmk2: Processing environment options: -comp=msvc
hbmk2 [hbssls]: Warning: Missing dependency: openssl
hbmk2 [hbssls]: Hint: Point envvar HBMK_WITH_OPENSSL or HB_WITH_OPENSSL to the
directory containing header 'openssl\ssl.h'
hbmk2 [hbssls]: Hint: Project URL(s): https://www.openssl.org/
https://www.openssl.org/community/binaries.html