I haven't had a proper internet connection for almost 3 weeks (I moved house), anyway I'm trying to compile the source code the same way I had before, which is using the previous method as opposed to using the new buildtools installer and the error I get is:

Generating m4510 source file...
Generating m6502 source file...
Generating m65c02 source file...
Generating m65ce02 source file...
Generating m6509 source file...
Generating m6510 source file...
Generating n2a03 source file...
Generating r65c02 source file...
Generating m740 source file...
Generating m6809 source file...
Generating hd6309 source file...
Generating konami source file...
Generating TMS57002 source file...
m68kmake.c
Linking build/generated/m68kmake.exe...
Generating M68K source files...

Musashi v4.90 680x0, CPU32, and ColdFire emulator
Copyright Karl Stenerud and the MAME team.

Generated 1972 opcode handlers from 523 primitives
makefile:352: *** MINGW64 is not set. Stop.

MingW64 was set before, so I don't know why it's not set any more. My set path looks like this:

set path=C:\mingw\mingw64-w64\bin;C:\mingw\mingw64-w64\opt\bin;C:\mingw\mingw64-w64\Qt\bin;

I suspect I need to add more paths? The set paths above are the ones listed on the MAME Dev website.

Any help would be great.