Allow adding per-package override rsync exclusions

Allow adding per-package override rsync exclusions

This allows using <PKG>_SRCDIR_OVERRIDE_RSYNC_EXCLUSIONS in local.mk to
skip copying parts of source trees unneeded for building. For example,
when developing WebKitGTK+, it's handy to skip copying all the tests and
other build directories, which are huge:

    WEBKITGTK_OVERRIDE_SRCDIR = /home/aperez/WebKit
    WEBKITGTK_OVERRIDE_SRCDIR_RSYNC_EXCLUSIONS = \
        --exclude JSTests --exclude ManualTests \
	--exclude PerformanceTests --exclude WebDriverTests \
	--exclude WebKitBuild --exclude WebKitLibraries \
	--exclude WebKit.xcworkspace --exclude Websites \
	--exclude Examples

This saves a good chunk of time when rsync is used for the first time to
copy the source tree over before building.

Signed-off-by: Adrian Perez de Castro <aperez@igalia.com>
[Arnout: move documentation to the end of the section]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>