Newer versions of MSVC are unhappy with the strategy of the build
environment redefining "inline" (even though they don't support the
actual keyword). Instead we define OPUS_INLINE to the right thing
in opus_defines.h.
This is the same approach we use for restrict.
The usage of restrict in include/opus_custom.h was missed
by the prior commit replacing this keyword with a macro.
Also fixes some prototype/function agreement with respect to
restrict.
In places where an ignored return or a null-arg is a sure indication
of a bug add the GCC warning attributes. The null arg annotation
is not enable for Opus itself because it will cause the compiler
to optimize out some null checks. I don't trust our callers
quite that much.
