Avoid the missing initializer warnings if the compiler allows us to

initialize unions.

Rev: src/acconfig.h:1.88
Rev: src/array.c:1.115
Rev: src/bignum.c:1.22
Rev: src/configure.in:1.530
Rev: src/encode.c:1.102
Rev: src/error.c:1.73
Rev: src/mapping.c:1.128
Rev: src/svalue.c:1.109