Bug#773329: Incorrect Cflags line in jack.pc

[Ted Felix]

Package: libjack-jackd2-dev
Version: 1.9.10+20140719git3eb0ae6a~dfsg-2

The cflags from pkg-config for jack are incorrect which breaks makefiles 
and configure scripts that depend on them (e.g. those for rosegarden).

The culprit is the Cflags line at the end of 
/usr/lib/x86_64-linux-gnu/pkgconfig/jack.pc.  This:

   Cflags: -I/usr/include

...should be this:

   Cflags: -I${includedir} -I${includedir}/jack

This problem is present in both jessie and sid.  I didn't check wheezy 
or prior.