cmake: Avoid path construction in pkg-config file

Alternative fix for: #563

It is not generally true that `CMAKE_INSTALL_<dir>` variables are relative paths:

https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files

Absolute paths will cause things like this to appear:

    includedir=${prefix}//nix/store/87zhscw1p08nqm73ls2sd82zmr1a8rni-libiio-0.21-dev/include

It would be best if the paths were relative to `${prefix}` pkg-config variable so that they could be overridden:

https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

but since that is not easy to do in CMake, we will just use absolute paths.

libiio does not expect other project to install artefacts to its installation paths so non-overriddable paths should no do much harm.

Signed-off-by: Jan Tojnar <[email protected]>

libiio.pc.cmakein

@@ -1,7 +1,7 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 
 Name: libiio
 Description: Library for interfacing IIO devices