Fix Clang 3.8.1 warnings

[jasp00]

This request deals with these Clang warnings:

• Shifting negative values.
• Possible truncations.
• Uninitialized variables.
• Self-assignments.
• Unused variables.
• Unsupported compiler option.

I add a shell function to configure.ac to check compiler options, but you may want to use AX_CHECK_COMPILE_FLAG from the Autoconf archive instead. In the XML files, I use an attribute to flag a list of unused variables.

[swh]

Thanks, merged