v8config.h: introduce V8_LIBC_MSVCRT

Use this for detecting MSVCRT library features instead of
V8_CC_MSVC.

One use case for this is when compiling with Clang together with the
MSVC library. In that case, V8_CC_MSVC will be false, but V8_LIBC_MSVCRT
will be true.

BUG=82385
LOG=n
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/145593003

Patch from Hans Wennborg <hans@chromium.org>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@18888 ce2b1a6d-e550-0410-aec6-3dcde31c8c00