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

src/tools/pg_bsd_indent/Makefile

@@ -25,6 +25,8 @@ OBJS = \
 	parse.o \
 	pr_comment.o
 
+$(OBJS): CFLAGS += $(PERMIT_MISSING_VARIABLE_DECLARATIONS)
+
 all: pg_bsd_indent
 
 pg_bsd_indent: $(OBJS) | submake-libpgport

src/tools/pg_bsd_indent/meson.build

@@ -18,6 +18,7 @@ endif
 
 pg_bsd_indent = executable('pg_bsd_indent',
   pg_bsd_indent_sources,
+  c_args: cflags_no_missing_var_decls,
   dependencies: [frontend_code],
   include_directories: include_directories('.'),
   kwargs: default_bin_args + {