Bug#961976: mumps: provide 64-bit build

[Drew Parsons]

To clarify this Bug, the current 64-bit build now provided for MUMPS 
(Bug#961185) means 64-bit PORD (64-bit ordering of indices), using the 
OPTC=-DPORD_INTSIZE64 flag. MUMPS_INT remains 32 bit.

This Bug (#961976) refers to a full all-integer 64-bit build, which 
would be triggered with OPTC=-DINTSIZE64 OPTF=-fdefault-integer-8 and 
would make MUMPS_INT 64 bit.

Drew