configure.ac: Simplify detection of Windows compilation

There are a few parts within the library where code is conditionally
compiled based on whether or not the target OS is Windows. Prior to
commit 8b09dd490d ("core: Kill the OS_* definitions and use in the
source code"), the OS_WINDOWS definition was used. With that definition
gone, the checks were replaced with _WIN32. Unfortunately the different
cross-platform toolchains do not universally define this, so add this to
AM_CPPFLAGS when building for Windows.

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>

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