Windows compilation fixes

A set of fixes to get FontForge compiling again on Windows.

* Removed plibc dependency in favour of using gnulib
* Resolved naming conflict with PrintDlg (introduced when gnulib was used, which included windows.h in more places, hence causing the issue)
* Added -no-undefined flag (via MY_LIB_LDFLAGS) to pyhook dlls
* Convert backslashes to slashes for file paths on Windows (this fixes the issue with the file/folder browser not being able to go up a directory initially)
* Default to the 'My Documents' folder for file browser instead of a user's home directory - this is the standard behaviour of pretty much all Windows applications.
* Don't save preferences directly to %APPDATA%, but instead create a folder specifically for FontForge