issue #17: avoid _Static_assert from c++

Merged Thomas E. Dickey requested to merge dickey/libxt:issue-17-no-static-assert into master

From Thomas Klausner, forwarding commit by Jonathan Perkin in pkgsrc, which said

libXt: Do not use _Static_assert in C++ code.

Fixes lots of dependencies.

Signed-off-by: Thomas E. Dickey dickey@invisible-island.net

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