> I would suggest to use 2^30 base only if sizeof(long)>=8 (64 bits
CPU).

Thats not the correct test.  Test for an actual 64-bit build target. 
sizeof(long) and sizeof(long long) are not usefully related to that in
any sort of cross platform manner.  On windows, we'd define the flag for
15 vs 30 bit longs in the build configs for the various build targets. 
On every thing else (autoconf), we can use a configure test to check the
same things that platform.architecture() checks to return '32bit' vs
'64bit'.