curlbuild.h.dist: check __LP64__ as well to fix MIPS build

The preprocessor check that sets up the 32bit defines for non-configure
builds didn't work properly for MIPS systems as __mips__ is defined for
both 32bit and 64bit. Now __LP64__ is also checked and indicates 64bit.

Reported-by: Tomas Jakobsson
Fixes #813