Detect endianness at configuration with Autoconf AC_C_BIGENDIAN feature

Currently IEEE_8087 (little endian) is defined as default in "jv_dtoa.c". From my understanding, to be able to use jq on big endian architecture you have to manually change this default define.

So to be able to use jq on bigendian architectures without changing code we need to be able to configure endianness. This patch does that.

FYI, compiling for bigendian, without changing the default define would result in numbers not being parsed correctly or even causing the jq application to hang.