Updated MinGW-32 and MinGW-64 for latest MinGW-W64 compiler
and VS2019. Ran 32- and 64-bit SNFMultiTest program.
Removed from 32bitDLL and 64bitDLL files created by the build procedure.
Added to 32bitDLL the needed DLLs, and removed old DLLs.
Built 32- and 64-bit DLLs on 64-bit Vista using MinGW-64 (as before) and MinGW. Ran BuildDistribution to create new SNFMultiSDK_Windows_3.1.zip. Added missing .exp and .lib files to SNFMultiSDK_Windows_3.1.