No, there isn't.
As I said before, there is no way to force any 'C' compiler to generate any specific machine instructions. That's the whole point.

However, the Keil C51 compiler definitely will use the DJNZ instruction when it can:
http://www.keil.com/discuss/docs/thread4066.htm
http://www.keil.com/discuss/docs/thread1082.htm

There must be something in your 'C' source that prevents it from doing so. You need to write your 'C' source code with due consideration of the underlying execution environment; ie, the 8051 architecture, instruction set, etc.

You will need to read both the Keil C51 Manual, and the 8051 "bible" to understand this fully.

The following documents are commonly referred to as "the bible" for the 8051:

http://www.semiconductors.philips.com/acrobat/v...ARCH_1.pdf
http://www.semiconductors.philips.com/acrobat/v...UIDE_1.pdf
http://www.semiconductors.philips.com/acrobat/v...WARE_1.pdf

The 80C51_FAM_PROG_GUIDE_1.pdf describes the 8051 Instruction Set.