qt5base: use system/buildroot provided freetype

Select (and use) builroot provided freetype in case qt5base gui is
selected (gui without freetype/font support makes not much sense
and gives compile errors).

Qt 5.6 needs some support to find the include headers, 5.8 is
clever enough inherently.

Fixes [1] (by avoiding the build-in freetype library which depends
on activated libpng support):

  Project ERROR: Library 'libpng' is not defined.
  make[3]: *** [sub-3rdparty-freetype-make_first] Error 3

[1] http://autobuild.buildroot.net/results/219162ba616289f799a5053c4dcc6b8574284283



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>