Avoid world-writable device

Group plugdev should be sufficient, access to the device will be controlled by
membership in it.
udev rules reside in /usr now

That is; those shipped in with the software. User can override them by
overlaying them with a file in /etc.
Allow overriding compiler flags

-g and -O flags should not be forced upon the user. Distributions often prefer
different flags: -O0 prevents compiler from generating information useful for
security features, such as -D_FORTIFY_SOURCE=2.

Libusb flags are made overridable as well, so that users can choose alternate
libusb installation or maybe a fork such as libusbx.
Nuke some unused variables
Don't let fread() fail silently
fuse_decl offset is an array index

Array indices are integers, not characters. Compiler complains.
Merge pull request #10 from lkundrak/lr-fedora

Various fixes/enhancements