gst-build does not work on macOS

Submitted by Olivier Crête @ocrete

Link to original bug (#781148)

Description

When I try to run "meson build" on osx in gst-build, I get:

Program g-ir-scanner found: YES (/usr/local/bin/g-ir-scanner)
Program g-ir-compiler found: YES (/usr/local/bin/g-ir-compiler)

Meson encountered an error in file subprojects/gstreamer/gst/meson.build, line 235, column 4:
gobject-introspection dependency was not found, gir cannot be generated.