Such kludges should be part of rntcl, not nettle, I hope?
For the record, the filenames aes.asm and aes.c are taken already, for the assembler version before processing by m4, and the plain C version, respectively.
(I'd actually like to support plain VC++ compilation on windows, without rntcl, using just GNU make (or even nmake, if it's capable enough) and command line tools like cl, and some special win32-configure script, but I need some good windows hacker to do most of the work for that).