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/interfaces/ecpg/test/Makefile.regress

@@ -3,7 +3,7 @@
 
 override CPPFLAGS := -I../../include -I$(top_srcdir)/src/interfaces/ecpg/include \
 	-I$(libpq_srcdir) $(CPPFLAGS)
-override CFLAGS += $(PTHREAD_CFLAGS)
+override CFLAGS += $(PTHREAD_CFLAGS) $(PERMIT_MISSING_VARIABLE_DECLARATIONS)
 
 LDFLAGS_INTERNAL += -L../../ecpglib -lecpg -L../../pgtypeslib -lpgtypes $(libpq)
 

src/interfaces/ecpg/test/meson.build

@@ -27,6 +27,7 @@ testprep_targets += pg_regress_ecpg
 
 # create .c files and executables from .pgc files
 ecpg_test_exec_kw = {
+  'c_args': cflags_no_missing_var_decls,
   'dependencies': [frontend_code, libpq],
   'include_directories': [ecpg_inc],
   'link_with': [ecpglib_so, ecpg_compat_so, ecpg_pgtypes_so],