Kind of hacky fix for banshee-2.6.0

[Stefan Wendler]

On Wednesday 04 December 2013 11:45:05 Mark Felder wrote:
> 
> I have a build dependency issue so I can't even get my poudriere to
> build banshee and test further. I'll poke at it again later this week if
> I get a chance. Feel free to bounce questions off us in IRC as well.
I will do that.

But I have a general question. Is it even possible to do the following within 
REINPLACE_CMD: insert a line, if it is not present? Because the 
ASSEMBLY_BUILD_FLAGS variable has to be in almost all Makefile.in files. And 
there are alot.

Cheers,
Stefan