configure.ac : Drop -Wunreachable-code.

Warns on things like a break after a return in a switch statement.
Doesn't seem that useful a warning.
Fix a couple of warnings uncovered by the clang compiler.
Add file Scripts/clang-sanitize.sh.
src/ : Fix compiler warnings related to use of MAKE_MARKER macro.

Warnings were from the Clang compiler, complaining about things like
'255 << 24' overflowing a 32 bit integer. Solution was to change some
ints to uint32_t and an casts in the MAKE_MARKER macro itself.