all: Fix license name

"GNU Public License" does not exist. It is called "GNU General Public
License", "GNU GPL" or simply "GPL".
libusb: Fix 'dev' memory leak on hid_init() failure.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
testgui: Remove pre-built testgui executable

Executables should have never been in the repo to begin with.
autotools: Add mac_support.h to Makefile.am

Not sure how this got missed.
HIDAPI 0.8.0-rc1