BUG #15190: Build configure should have option to disableclock_gettime on MacOS 10.12 and up.

The following bug has been logged on the website:

Bug reference:      15190
Logged by:          Marc Van Olmen
Email address:      marcvanolmen@gmail.com
PostgreSQL version: 10.3
Operating system:   MacOS 10.13
Description:

Steps to reproduce issue.

1. Use MacOS 10.12 or higher. I tested MacOS 10.13 because for latest Xcode
9.3 this is the minimal system.
2. Run the standard configure command and make and make install.
3. It will produce an executable for example ./lib/postgress that use the
function clock_gettime.
4. Trying to run this produced executable on macOS 10.10 or lower will
produce the following error

dyld: Symbol not found: _clock_gettime
  Referenced from:
/Volumes/Checkout/Checkout.app/Contents/Resources/postgres_10/bin/postgres
  Expected in: /usr/lib/libSystem.B.dylib

Proposed solution:

We should have an option that disable to the automatic search for using the
clock_gettime on MacOS 10.12 or higher


Other open source projects have also introduced similar options:

./configure --disable-clock-gettime  

https://github.com/libevent/libevent/issues/399
https://github.com/libevent/libevent/pull/400/files

thanks,

Marc Van Olmen