Fix some minor numerical precision issues.

Casts, type issues, theoretical truncation, sign comparisons... exposed
courtesy of MSVC's warnings.
Linker flags for Windows Perl build
Add .obj files to .gitignore
Always compile as C++ with MSVC

Although the generated XS is C89-compliant now, it must be compiled in
C++ mode like the rest of the code due to a mismatch between the sizes
of the C++ bool type and the emulated bool type.
Correct cfish library name on Windows
Update .gitignore files for MSVC