SNMP compilation failed because variables DS_LIBRARY_ID and DS_LIB_QUICK_PRINT were undeclared (see compilation error below).  These values are defined in default_store.h, but default_store.h was not included in snmp.c because HAVE_DEFAULT_STORE_H was undefined even though it existed.  Including default_store.h successfully compiled PHP with snmp support.  SNMP header files were installed with Red Hat's rpm ucd-snmp-devel-4.1.1-2.

SNMP.C CODE
#ifdef HAVE_DEFAULT_STORE_H
#include "default_store.h"
#endif

THE COMPILATION ERROR
snmp.c: In function `php_snmp':
snmp.c:225: warning: implicit declaration of function `snmp_synch_setup'
snmp.c: In function `php_if_snmp_get_quick_print':
snmp.c:357: warning: implicit declaration of function `ds_get_boolean'
snmp.c:357: `DS_LIBRARY_ID' undeclared (first use in this function)
snmp.c:357: (Each undeclared identifier is reported only once
snmp.c:357: for each function it appears in.)
snmp.c:357: `DS_LIB_QUICK_PRINT' undeclared (first use in this function)
snmp.c: In function `php_if_snmp_set_quick_print':
snmp.c:369: warning: implicit declaration of function `ds_set_boolean'
snmp.c:369: `DS_LIBRARY_ID' undeclared (first use in this function)
snmp.c:369: `DS_LIB_QUICK_PRINT' undeclared (first use in this function)