Use static/inline where appropriate, switch to C99

fdupes contains many extremely small functions that are called in
few places and can benefit from explicit inlining. Functions
should also be declared "static" where appropriate to allow for
better compiler optimization. Added "static" and "inline" where
appropriate.

Removed "long long" references, replacing with proper C99 types.
Changed Makefile to use C99, higher optimization, and pedantic
compiler warnings. Added headers needed for C99 types.