Hello Johan,
You can check the value of such system variables in report RSPARAM (execute through SA38 or SE38). You'll see there that for windows:
DIR_SEP: \
FT_DLL_PREFIX: <blank, no value>
FT_DLL: .dll
Therefore, this holds true:
sapcrypto.dll = $(FT_DLL_PREFIX)sapcrypto$(FT_DLL)
I hope this clarifies.
Best Regards,
Guilherme de Oliveira
SAP Active Global Support