libusb/Makefile.am: Remove unnecessary assignments
Remove the unnecessary assignment of EXTRA_DIST. Anything that could end
up in SOURCES is automatically included in the distribution.
Remove the unnecessary assignment of libusb_1_0_la_CFLAGS. AM_CFLAGS is
already the default value, but assigning to the variable makes automake
think that custom CFLAGS are needed and therefore causes a bunch of
pointless additional rules to be generated. As a reference, the
generated Makefile shed 21,560 bytes (from 52,029 bytes to 30,469).
Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>