0.5.3 (the downloaded tarball) doesn't build on Solaris 9. Two minor changes are required. Pardon me for not creating a Unix patch; it would take me too long to refresh myself about how to do it.
src/rxvt.h, line 58:
src.init.c, line 811: insert
#define NAME_MAX 255
I haven't tested the resulting build thoroughly, but it does build without incident and starts up. The sample config file in share/mrxvt.sample appears to work.