Makefile.package.in: Makes it possible to override the default extract commands

When using GENTARGETS (or macroes depending on it), there is no way of
specifying a custom extraction procedure. When using the patch one can
simply define $(PACKAGE_NAME)_EXTRACT_CMDS which will override the
default. If non is defined it will fall back to the default extract
procedure.

An example could look like this:

=== PACKAGE BEGIN ===
FGLRX_VERSION=11-3
FGLRX_SOURCE:=ati-driver-installer-$(FGLRX_VERSION)-x86.x86_64.run
FGLRX_SITE:=http://www2.ati.com/drivers/linux/



define FGLRX_EXTRACT_CMDS
	sh $(DL_DIR)/$(FGLRX_SOURCE) --extract $(@D)
endef

$(eval $(call GENTARGETS,package,fglrx))
=== PACKAGE END ===

Signed-off-by: Allan W. Nielsen <a@awn.dk>
Cc: Allan W. Nielsen <a@awn.dk>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>