gcc: Allow our C++ headers to survive -Wsystem-headers (2).

The previous commit (894c19b2d84fd28593ce799678d7d9f7d33936ef) wasn't
actually working because __LONG_LONG_SUPPORTED wasn't yet known.

In

gcc: Allow our C++ headers to survive -Wsystem-headers (2).

The previous commit (894c19b2d84fd28593ce799678d7d9f7d33936ef) wasn't
actually working because __LONG_LONG_SUPPORTED wasn't yet known.

Include <sys/cdefs.h> for it.

Reported-by: tuxillo

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gcc: Allow our C++ headers to survive -Wsystem-headers.

It was breaking on defining a macro that resolves to a defined()
directive, which is apparently an issue in C++ land. As a workaround,
define

gcc: Allow our C++ headers to survive -Wsystem-headers.

It was breaking on defining a macro that resolves to a defined()
directive, which is apparently an issue in C++ land. As a workaround,
define a C++ specific macro that is either 0 or 1 for long long
support and can be used without issues.

Stop filtering out -Wsystem-headers in CXXFLAGS. The remaining -W...
options that are filtered out are all C specific.

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