pgsql: Add, optional, support for 128bit integers.

Add, optional, support for 128bit integers.

We will, for the foreseeable future, not expose 128 bit datatypes to
SQL. But being able to use 128bit math will allow us, in a later patch,
to use 128bit accumulators for some aggregates; leading to noticeable
speedups over using numeric.

So far we only detect a gcc/clang extension that supports 128bit math,
but no 128bit literals, and no *printf support. We might want to expand
this in the future to further compilers; if there are any that that
provide similar support.

Discussion: 544BB5F1(dot)50709(at)proxel(dot)se
Author: Andreas Karlsson, with significant editorializing by me
Reviewed-By: Peter Geoghegan, Oskari Saarenmaa

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8122e1437e332e156d971a0274879b0ee76e488a

Modified Files
--------------
config/c-compiler.m4 | 37 +++++++++++++++++++++++++++++
configure | 52 +++++++++++++++++++++++++++++++++++++++++
configure.in | 3 +++
src/include/c.h | 11 +++++++++
src/include/pg_config.h.in | 3 +++
src/include/pg_config.h.win32 | 3 +++
6 files changed, 109 insertions(+)