Remove superflous checks in configure.ac.

Removing checks for memset, malloc, free, stdlib.h, and string.h as they are always available.
Corrected typo from include/webvtt/util.h:81

Replaced 'typ4edef' with 'typedef'
Change #warn to #warning in include/webvtt/util.h

MSVC should not be used if that branch is encountered, and gcc implements
a #warning, so perhaps if the compiler is unknown, #warning will be more
acceptable than #warn
Merge pull request #37 from RickEyre/issue-10

Remove superflous checks in configure.ac.
Merge pull request #38 from caitp/issue-38

Syntax error in util.h
Remove unused AC_FUNC_MALLOC check.

This macro is about enabling an rpl_malloc wrapper if the platorm
malloc(0) doesn't return a valid pointer. I don't know if we're
depending on that behaviour, but we're not providing a replacement.