F clang wall wextra compilation fixes

these commit allow compilation with clang at the highest warning level

$ clang --version
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix

$ clang -Wall -Wextra src/*.c 3rdparty/*.c -I./3rdparty/ -c
$