Potentially bumping the build requirement to C99

[Vincent Rabaud]

Hi all,

WebP is mostly C89 with bits of C99 (like // comments). We cannot switch to C11 as Microsoft Visual Studio does not support it fully. But we could go with C99.

Pros on requiring C99 by default:
- clean-up some code (conditions on defining "inline", integer types from inttypes.h).
- more coding flexibility with intermingled declarations and code.

- maybe optimizations (probably very small, but there are more functions available like log2).

Cons:

- less platforms supported: for Microsft Visual Studio, 2013 or later would be required; gcc should not be older than 4.0

Are there any configurations out there that would be problematic? Thx.