Small warning during compile

[Rik]

I'm getting this new warning during compilation:

libinterp/parse-tree/lex.cc:16:32: warning: unknown option after ‘#pragma
GCC diagnostic’ kind [-Wpragmas]

The line itself is

#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"

I suppose my gcc version, 5.4.0, is too old to recognize this particular
warning ID.

We can simply live with it, or maybe this particular pragma needs to be
protected with and ifdef depending on whether the compiler has this
warning.  It wouldn't be that hard to put in a test in configure.ac to set
an appropriate #define definition.

--Rik