I've never used Turbo C... Searching through Google, though, it appears that it is old and there isn't a way to produce DLLs with it (directly) since it was only made for producing DOS programs and not Windows programs.
I have used the MinGW C compiler to create DLLs for JNI before. There's even an entry in their FAQ specifically on how to create a DLL for JNI.
Not being familiar with Turbo C, or the program you're trying to create a JNI interface to, I'm not sure how much of a problem it would be to port your program from Turbo C to MinGW, but this Turbo C to GCC Porting Tool might be helpful. (GCC code should work in MinGW, too.)