getgrouplist is available in uClibc since 0.9.30 (s. http://git.uclibc.org/uClibc/commit/libc/pwd_grp/pwd_grp.c?id=9385a9419d98aeb56fe54b7452d1310ef20cea95)

check if it's available and provide a replacement if necessary

--- config.h.in
+++ config.h.in
@@ -21,6 +21,9 @@
 /* Define to 1 if you have the `getgrnam_r' function. */
 #undef HAVE_GETGRNAM_R
 
+/* Define to 1 if you have the `getgrouplist' function. */
+#undef HAVE_GETGROUPLIST
+
 /* Define to 1 if getgrouplist() takes ints as arguments */
 #undef HAVE_GETGROUPLIST_TAKES_INTS
 
--- configure.ac
+++ configure.ac
@@ -34,7 +34,7 @@
 
 dnl Prefer thread-safe functions, if available
 AC_CHECK_FUNCS([getgrgid_r getgrnam_r gethostbyname_r getpwnam_r getpwuid_r  \
-                openpty strcasestr getresuid getresgid setresuid setresgid ])
+                openpty strcasestr getresuid getresgid setresuid setresgid getgrouplist ])
 
 dnl We prefer ptsname_r(), but will settle for ptsname() if necessary
 AC_TRY_LINK([#ifndef _XOPEN_SOURCE
--- configure
+++ configure
@@ -10655,7 +10655,7 @@
 
 
 for ac_func in getgrgid_r getgrnam_r gethostbyname_r getpwnam_r getpwuid_r  \
-                openpty strcasestr getresuid getresgid setresuid setresgid
+                openpty strcasestr getresuid getresgid setresuid setresgid getgrouplist
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"
--- shellinabox/launcher.c
+++ shellinabox/launcher.c
@@ -170,6 +170,47 @@
 static int   launcher = -1;
 static uid_t restricted;
 
+#if !defined(HAVE_GETGROUPLIST)
+/*
+ * define own getgrouplist
+ * based on the freebsd implementation (s. http://svnweb.freebsd.org/base/stable/9/lib/libc/gen/getgrouplist.c?revision=165903&view=markup)
+ * modified to support calls like getgrouplist(*, *, NULL, *)
+ */
+int
+getgrouplist(const char *uname, gid_t agroup, gid_t *groups, int *grpcnt)
+{
+	const struct group *grp;
+	int i, maxgroups, ngroups;
+
+	maxgroups = *grpcnt;
+
+	if (groups)
+		groups[0] = agroup;
+	ngroups = 1;
+
+	/*
+	 * Scan the group file to find additional groups.
+	 */
+	setgrent();
+	while ((grp = getgrent()) != NULL) {
+		if (grp->gr_gid == agroup)
+			continue;
+		for (i = 0; grp->gr_mem[i]; i++) {
+			if (strcmp(grp->gr_mem[i], uname) == 0) {
+				if (groups && ngroups < maxgroups)
+					groups[ngroups] = grp->gr_gid;
+				ngroups++;
+				break;
+			}
+		}
+	}
+	endgrent();
+
+	*grpcnt = ngroups;
+	return (ngroups > maxgroups) ? -1 : ngroups;
+}
+#endif
+
 // MacOS X has a somewhat unusual definition of getgrouplist() which can
 // trigger a compile warning.
 #if defined(HAVE_GETGROUPLIST_TAKES_INTS)