Remove an unused variable
Remove some unnecessary ifdefs

These headers are standard headers that are available in MSVC as well.
There's nothing ifdeffed in the implementation of this file that would
explain why these headers only are required in these configurations.
Make the first C function parameter const void* in c_interface_test

This fixes warnings when building with MSVC.
Remove a duplicate -MD declaration
Move all x86 vs arm handling to msvc-common
Rename the msvc-arm platform file to msvc-wp

The architecture handling is now decoupled from the main
platform file. Now you can build for Windows Phone for x86 (for
the emulator) by "make OS=msvc-wp ARCH=x86".
Merge pull request #673 from mstorsjo/msvc-common-arch

Move all arch handling in msvc platform files into msvc-common
Merge pull request #669 from mstorsjo/remove-unnecessary-ifdef

Remove some unnecessary ifdefs
Merge pull request #667 from mstorsjo/remove-unused-variable

Remove an unused variable
Merge pull request #671 from mstorsjo/c-api-test-warnings

Make the first C function parameter const void* in c_interface_test