Fix building minidump processor and minidump tools for MinGW

Part of https://breakpad.appspot.com/548002 was commited as svn r1399 to fixed
client compilation for MinGW

This is mainly the configure.ac and Makefile.am changes left over from
https://breakpad.appspot.com/548002/ with a bit of updating, to fix building of
the minidump processor library and tools for MinGW

- Link minidump_stackwalk, minidump_dump, microdump_stackwalk with PTHREAD_LIBS
as pthread_cancel is used
- Link minidump_stackwalk, microdump_stackwalk with ws2_32 as ntoh functions are
used
- _s function variants should only be used if _MSC_VER, use _r variants with
MinGW (and define _POSIX to ensure they are prototyped)
- Don't try to include arpa/inet.h on Windows
- Provide int16_t, int64_t types

Note that a recent MinGW-w64 with a real strtok_r, rather than one emulated
using strtok is needed for the processor to work properly.