I'm trying to compile on an HP machine and I don't have gcc there. So I put CC=cc in the Local/Makefile, but I got this error:

(Bundled) cc: "buildconfig.c", line 54: error 1705: Function prototypes
  are an ANSI feature.

The bundled compiler is not an ANSI C compiler. You either have to get a copy of gcc from the HPUX Software Porting Archives or buy the ANSI cc from HP. The advice given by one user of HP systems on the Exim mailing list was as follows: Personally, I wouldn't use anything but the ANSI C compiler. gcc works for compilation, but it doesn't know squat about PA-RISC chips past the 1.0 rev. Since then, HP has come out with PA-RISC 1.1, 2.0, and 2.1, each with better features. gcc will compile for them, but it doesn't produce anywhere near the optimization that HP's compiler does. I took the gcc road when we moved from FreeBSD to HP-UX because I was familiar with it. After 6 months, I had to go and re-port everything over when we realized that gcc wasn't going to do it for us long-term. If I could give advice to any new HP-UX admin: don't use gcc if you can afford the ANSI C compiler. Based on the cost of even the lowest HP workstation, that usually isn't a problem.