With following diff of Makefile,

root@600m:/export/home/tjyang/Fossil-076f7adff2# diff Makefile Makefile.orig
39c39
< # LIB += -lsocket
---
> # LIB += lsocket
41c41
< LIB += -lsocket -lnsl
---
> # LIB += -lsocket -lnsl
root@600m:/export/home/tjyang/Fossil-076f7adff2#

I was able to build fossil on opensolaris b124.

root@600m:/export/home/tjyang/Fossil-076f7adff2# uname -a
SunOS 600m 5.11 snv_122 i86pc i386 i86pc Solaris
root@600m:/export/home/tjyang/Fossil-076f7adff2# cat /etc/release
                       OpenSolaris Development snv_122 X86
           Copyright 2009 Sun Microsystems, Inc.  All Rights Reserved.
                        Use is subject to license terms.
                            Assembled 28 August 2009
root@600m:/export/home/tjyang/Fossil-076f7adff2# ls -l fossil
-rwxr-xr-x 1 root root 1582864 2009-10-13 23:37 fossil
root@600m:/export/home/tjyang/Fossil-076f7adff2#