[subset] Remove test-subset, testing is planned to be done at the table level here with test/subset covering the complete subsetting operation.

When linking test-unicode statically it needs $(ICU_LIBS)
which contains all required flags.
Especially -lstdc++.

Fixes:
http://autobuild.buildroot.net/results/210/2107f9dfb39eeb6559fb4271c7af8b39aef521ca/Signed-off-by: NRomain Naour <romain.naour@openwide.fr>

We don't use gthread anymore, remove leftovers.

As reported by Peter Breitenlohner:

I think this is a very bad idea because ENV is used to specify a startup
file to be read by some/all shells.

Fails now.  Fixing.

New shaping testsuite and framework coming.

Remove hb_ft_get_font_funcs() as it cannot be used by the user anyway.

Add hb_ft_font_set_funcs().  Which will make the font internally use
FreeType.  That is, no need for the font to have created using the
hb-ft API.  Just create using hb_face_create()/hb_font_create() and
then call this on the font (after having set font scale).  This
internally creates an FT_Face and attached to the font.