Had to make some minor modifications to get SpiderMonkey 1.7.0 to compile on my computer (Mac OS X 10.6.8).

sm-on-osx.patch

diff -rupN js/src/config/Darwin.mk js-mine/src/config/Darwin.mk
--- js/src/config/Darwin.mk	2007-02-05 11:24:49.000000000 -0500
+++ js-mine/src/config/Darwin.mk	2011-11-23 13:49:20.000000000 -0500
@@ -46,7 +46,7 @@
 CC = cc
 CCC = g++
 CFLAGS +=  -Wall -Wno-format
-OS_CFLAGS = -DXP_UNIX -DSVR4 -DSYSV -D_BSD_SOURCE -DPOSIX_SOURCE -DDARWIN
+OS_CFLAGS = -DXP_UNIX -DSVR4 -DSYSV -D_BSD_SOURCE -DPOSIX_SOURCE -DDARWIN -DHAVE_VA_COPY
 
 RANLIB = ranlib
 MKSHLIB = $(CC) -dynamiclib $(XMKSHLIBOPTS) -framework System
diff -rupN js/src/jsprf.c js-mine/src/jsprf.c
--- js/src/jsprf.c	2006-07-06 22:12:02.000000000 -0400
+++ js-mine/src/jsprf.c	2011-11-23 13:47:45.000000000 -0500
@@ -57,7 +57,7 @@
 ** and requires array notation.
 */
 #ifdef HAVE_VA_COPY
-#define VARARGS_ASSIGN(foo, bar)        VA_COPY(foo,bar)
+#define VARARGS_ASSIGN(foo, bar)        va_copy(foo,bar)
 #elif defined(HAVE_VA_LIST_AS_ARRAY)
 #define VARARGS_ASSIGN(foo, bar)        foo[0] = bar[0]
 #else

Variation of instructions here: http://wiki.apache.org/couchdb/Installing_SpiderMonkey
Note: I usually install non-standard stuff in /opt/name-of-software-x.y.z

tar zxvf js-1.7.0.tar.gz
wget --no-check-certificate "https://gist.github.com/raw/1389554/f9e32f2d80b5248fd4e65ebd29dde4e586ce53cc/sm-on-osx.patch"
cd js
patch -p1 < ../sm-on-osx.patch
make BUILD_OPT=1 -f Makefile.ref
sudo make BUILD_OPT=1 JS_DIST=/opt/spidermonkey-1.7.0 -f Makefile.ref export