Currently when GTK3 is compiled without G_ENABLE_DEBUG, the inspector
can't be opened with GTK_DEBUG=interactive because it doesn't parse
this env var without G_ENABLE_DEBUG.
Since the inspector is always good to have, this commit now always
parses the GTK_DEBUG env var but only keep the "interactive" flag if
G_ENABLE_DEBUG isn't defined.
0a9e2d6ed7
