pkg-config: prefix sysroot path

[Kwiboo]

This PR makes cmake set sysroot prefixed <package>_INCLUDE_DIRS and other variables when a package is found by pkg-config.

This adds a patch to pkg-config that changes the following behaviors:

• System include and library paths is included in output for target pkg-config, PKG_CONFIG_ALLOW_SYSTEM_(CFLAGS|LIBS)
• No longer adds a second sysroot prefix when .pc-file already contains a sysroot prefixed path
• Sysroot path is prefixed for static library files
• Sysroot path is prefixed for includedir and libdir variables, they are usually added to cmake <package>_(INCLUDE_DIRS|LIBRARIES)

Needs more build and runtime testing for non-cmake packages.

[MilhouseVH]

I've clean built RPi/RPi2/Generic with this on LE master + Kodi master, and all seems well - no build failures, no missing or extra files, systems boot OK etc.

Edit: All of the builds included all PVR and inputstream addons.

[lrusak]

Jenkins is happy, I think we can shove this in

[MilhouseVH]

No issues from me, but this is currently labelled LE9.

[CvH]

@MilhouseVH feel free to change :)

[MilhouseVH]

Removed LE9 label.

[MilhouseVH]

RPi/RPi2/Generic all seem fine.

[MilhouseVH]

Added my approval based on my testing, but will leave the button pushing to chewitt as this will affect the beta. If chewitt doesn't want this for LE8 then we can add back the LE9 label.