Add -Wmissing-variable-declarations to the standard compilation flags

Add -Wmissing-variable-declarations to the standard compilation flags

This warning flag detects global variables not declared in header
files.  This is similar to what -Wmissing-prototypes does for
functions.  (More correctly, it is similar to what
-Wmissing-declarations does for functions, but -Wmissing-prototypes is
a superset of that in C.)

This flag is new in GCC 14.  Clang has supported it for a while.

Several recent commits have cleaned up warnings triggered by this, so
it should now be clean.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org