Retire COMPILER_MSVC in favour of _MSC_VER and _WIN32.

Tidy up util/pcre.cc very slightly while I'm here.

Fixes #52.

Change-Id: I67854772b7cc995cc63344d4b746e9f3d0db1949
Reviewed-on: https://code-review.googlesource.com/3495
Reviewed-by: Paul Wankadia <junyer@google.com>