pgsql: Try to detect runtime unavailability of __builtin_mul_overflow(i

Try to detect runtime unavailability of __builtin_mul_overflow(int64).

On some systems the results of 64 bit __builtin_mul_overflow()
operations can be computed at compile time, but not at runtime. The
known cases are arm buildfar animals using clang where the runtime
operation is implemented in a unavailable function.

Try to avoid compile-time computation by using volatile arguments to
__builtin_mul_overflow(). In that case we hopefully will get a link
error when unavailable, similar to what buildfarm animals dangomushi
and gull are reporting.

Author: Andres Freund
Discussion: https://postgr.es/m/20171213213754.pydkyjs6bt2hvsdb@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/c04d35f442a8c4fd5a20103b31839ec52fce3046

Modified Files
--------------
config/c-compiler.m4 | 12 ++++++++----
configure | 4 +++-
2 files changed, 11 insertions(+), 5 deletions(-)