Support config with mingw cross-compiled on Linux

prefer lower-case header and dll names
Bump Win32 API level to Windows 2003 0x502
Fix Win32 compile errors in disk_stats
Rename stat_type enumerations

Rename stat_type enumerations in example program to not conflict with
builtin Windows types

Windows defines typedefs for INT, FLOAT, etc. that conflight with the
stat_type enumeration. Add STAT_TYPE to these so they can build on
Windows (tested in mingw on Ubuntu 13.10)
Update win32 error handling
Update user_stats error handling on win32
Implement sg_get_sys_page_size on win32
Fix various win32 build errors in os_info
Fix various win32 build errors with network_stats
Fix various win32 build errors with memory_stats
Fix build errors if pthreads are not present
Resurrect CPU counters for Win32

Performance Data Helpers may be nonsense, but are better than nothing
Don't modify behaviour on non-Windows platforms.
Prepare for 0.91 release.

This includes a note in README that covers the RPATH/ldconfig issue
found #37, and also a note for the parallel installation discussion in
issue #25.