Fix running mbrola on Windows

[rhdunn]

This involves:

1. Factoring out the path resolution to a helper method (environment variable, registry key+value, user path, default path).
2. Using (1) to locate the mbrola binary (on POSIX) or DLL (on Windows).
3. Use (1) to locate the mbrola voices.

This should make it easier to use the installed mbrola binaries on Windows.

[rhdunn]

• Bulid the MBROLA voice files from the Windows solution file.
• Test using an MBROLA voice, fixing any issues.
• Update the documentation on installing and configuring mbrola (move that part to the readme file).

NOTE: This only works with a 32-bit version of espeak-ng. That loads the mbrola DLL correctly, so the loading logic does not need to change on that setup. Supporting mbrola on 64-bit espeak-ng is more complex, as it would require creating a 32-bit COM DLL to call the mbrola APIs from, then using the mbrowrap code to call the COM DLL on 64-bit Windows. That is out of the scope for this issue -- people wanting mbrola voices on Windows should be directed to the 32-bit version of espeak-ng.