UNDEFINED + ({}) and `+= implemented

Rev: src/docode.c:1.44
Rev: src/lex.c:1.62
Rev: src/modules/Gmp/mpz_glue.c:1.38
Rev: src/operators.c:1.46
Rev: src/program.c:1.109
Rev: src/program.h:1.50
Rev: src/security.c:1.10
Rev: src/testsuite.in:1.146