plugin: API: GST_PLUGIN_DEPENDENCY_FLAG_PATHS_ARE_RELATIVE_TO_EXE

plugin: API: GST_PLUGIN_DEPENDENCY_FLAG_PATHS_ARE_RELATIVE_TO_EXE

When a plugin declares a dependency using this flag, all the
relative paths are considered to be relative to the path of
the main executable.

We try to determine the path of the executable portably,
with implementations provided for Linux, Windows and Mac.

If retrieval of the path fails, we will not detect changes.

In order for the main executable path to be the same when
scanning a plugin in a child process, a new variable is
exposed in gst_private.h, _gst_executable_path

https://bugzilla.gnome.org/show_bug.cgi?id=788152