Using -Werror in CFLAGS make ./configure to fails with:
checking whether the C compiler works... no
configure: error: in `/home/travis/build/LudovicRousseau/libusb':
configure: error: C compiler cannot create executables
See `config.log' for more details
-Werror had no effect because CFLAGS was _not_ exported.
# should be removed and the code fixed
CFLAGS+=" -Wno-incompatible-pointer-types-discards-qualifiers"
-# fails on warning
-CFLAGS+=" -Werror"
+export CFLAGS
./autogen.sh