Fix gmp build for 32bit ubuntu.

Need to set ABI=32 when calling ./configure, at least for 32bit Ubuntu 14.04 desktop.

Without this setting, the configure step reports this error:

```
checking size of mp_limb_t... 4
configure: error: Oops, mp_limb_t is 32 bits, but the assembler code
in this configuration expects 64 bits.
You appear to have set $CFLAGS, perhaps you also need to tell GMP the
intended ABI, see "ABI and ISA" in the manual.
make: *** [gmp-config] Error 1
```