macOS snapshot: statically link to `libmatio` and dependencies, `gsl`, and `gcc` libraries

* The macOS snapshot until this point did not work unless the user had already installed `libmatio`, `gsl`, and `gcc` via Homebrew.
* We are not modifying `m4/ax_matio.m4` as we want to still be able to compile against shared libraries in development
11 jobs for master in 103 minutes and 54 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #8003
build_binaries

00:12:28

passed #8004
build_doc

00:01:49

 
  Test And Pkg
passed #8007
macOS
pkg_macOS

00:10:58

passed #8005
pkg_source

00:02:01

passed #8006
pkg_windows

00:25:20

passed #8011
test_dynare++

00:11:45

passed #8008
test_matlab

01:28:31

manual #8010
allowed to fail manual
test_octave
manual #8009
allowed to fail manual
test_old_matlab
 
  Deploy
passed #8012
restricted
deploy_manual_unstable

00:00:17

passed #8013
restricted
deploy_snapshot_unstable

00:00:34