Enable "-Wextra" compiler option and fix code to remove warnings

"-Wextra" option turns on the following warnings:
 -Wclobbered
 -Wempty-body
 -Wignored-qualifiers
 -Wmissing-field-initializers
 -Wmissing-parameter-type (C only)
 -Wold-style-declaration (C only)
 -Woverride-init
 -Wsign-compare
 -Wtype-limits
 -Wuninitialized
 -Wunused-parameter (only with -Wunused or -Wall)
 -Wunused-but-set-parameter (only with -Wunused or -Wall)
Remove invalid use of register

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>