lz4c : automatic output name when decoding a *.lz4 file, as requested by Peter Humphreys

Cleaner C99 detection, thanks to Elad
Modified Basic Types, as recommended by Wayne Scott
Simplified compression function
Corrected : warning on missing #undef in lz4hc, thanks Arseny Kapoulkine
Modified length loops as suggested by Wouter Vermaelen
Modified pragma pack directive for older versions of GCC, as reported by Peter Costello
Updated xxHash to r29
Indent lz4hc.c with spaces