C89 Compatibility

[chr-1x]

There are three things using C99 features:

• twc_option and twc_buffer construction macros
• exactly one other macro construction somewhere in twitter.c
• declare-anywhere used here and there

None of these are critical, and use of the former needs to be scoped down a lot anyway for ease of language binding. C89 compat is a reasonable target and likely opens up path for compiling on less recent compilers (e.g. older versions of MSVC)

[nothings]

I may try to make this change at some point, but no promises.