Don't use ln -r (relative) option for 'symlinks' build target

As reported by Pasta68K:
FreeBSD's ln doesn't support -r

ln -sfr clang.freebsd..exe.release/* /sbbs/exec
ln: illegal option -- r

So use realpath instead. Hopefully all the targeted *nix platforms include
'realpath'.

src/sbbs3/targets.mk

@@ -110,9 +110,9 @@ install: all
 
 .PHONY: symlinks
 symlinks: all
-	ln -sfr $(EXEODIR)/* $(SBBSEXEC)
-	ln -sfr $(LIBODIR)/* $(SBBSEXEC)
-	ln -sfr */$(EXEODIR)/* $(SBBSEXEC)
+	ln -sf `realpath $(EXEODIR)/*` $(SBBSEXEC)
+	ln -sf `realpath $(LIBODIR)/*` $(SBBSEXEC)
+	ln -sf `realpath */$(EXEODIR)/*` $(SBBSEXEC)
 endif
 
 .PHONY: FORCE