modifications for compiling mex on macOS

* only support 64 bit mex
* check whether local compiler exists; if not use system compiler; if that doesn't exist stop processing
* move to minimum macOS 10.9, corresponding to the MATLAB mex min
6 jobs for master in 3 minutes and 19 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #9699
build_linux_32

00:00:39

passed #9700
build_linux_64

00:01:26

passed #9703
macOS
build_macOS

00:02:09

passed #9701
build_windows_32

00:01:28

passed #9702
build_windows_64

00:01:41

 
  Prepare
passed #9704
aggregate_builds

00:00:10