Improved endianess check, fixed compiler warnings

[eleccoder]

• __BYTE_ORDER__ evaluated to also work with gcc ARM (cross) compiler, for example
• Usage of preprocessor macros improved; could now be compiled w/o warnings with '-Wundef'
• Fixed compiler warnings seen on Windows/MinGW by using stdint types:
  wav.c:434:108: warning: cast from pointer to integer of different size
  [-Wpointer-to-int-cast]
  self->format_chunk.header.size =
  (WavU32)((WavUIntPtr)&self->format_chunk.body.ext_size -
  (WavUIntPtr)&self->format_chunk.body);
  ^
• Obsolete comment removed

[brg1ng]

Merged with modification.