build: Compile with -Wdiscarded-qualifiers.

build: Compile with -Wdiscarded-qualifiers.

This option is unsupported (but warnings are invariably enabled) for
old gcc versions including 4.2.2. It is supported but not enabled by
default for vanilla gcc-5.5.0, but is enabled for the gcc versions
which ship with Ubuntu-14.04 (4.8.4) and Ubuntu-16.04 (5.4.0).

This patch enables the warning everywhere. We have to employ the
cc-option macro to determine whether the option is supported.