Add warning option -Wold-style-declaration

This warning has been triggered a few times via the buildfarm (see
commits 8212625e53, 2b7259f855, afe86a9e73), so we might as well
add it so that everyone sees it.

(This is completely separate from the recently added
-Wold-style-definition.)

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/aa73q1aT0A3/vke/%40ip-10-97-1-34.eu-west-3.compute.internal

configure

@@ -5576,6 +5576,47 @@ fi
 
 
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wold-style-declaration, for CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wold-style-declaration, for CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wold_style_declaration+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${CFLAGS} -Wold-style-declaration"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_CC_cflags__Wold_style_declaration=yes
+else
+  pgac_cv_prog_CC_cflags__Wold_style_declaration=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+CC="$pgac_save_CC"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CC_cflags__Wold_style_declaration" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wold_style_declaration" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wold_style_declaration" = x"yes"; then
+  CFLAGS="${CFLAGS} -Wold-style-declaration"
+fi
+
+
+  # -Wold-style-declaration is not applicable for C++
+
   # To require fallthrough attribute annotations, use
   # -Wimplicit-fallthrough=5 with gcc and -Wimplicit-fallthrough with
   # clang.  The latter is also accepted on gcc but does not enforce

configure.ac

@@ -555,6 +555,8 @@ if test "$GCC" = yes -a "$ICC" = no; then
   PGAC_PROG_CXX_CFLAGS_OPT([-Werror=unguarded-availability-new])
   PGAC_PROG_CC_CFLAGS_OPT([-Wmissing-format-attribute])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wmissing-format-attribute])
+  PGAC_PROG_CC_CFLAGS_OPT([-Wold-style-declaration])
+  # -Wold-style-declaration is not applicable for C++
 
   # To require fallthrough attribute annotations, use
   # -Wimplicit-fallthrough=5 with gcc and -Wimplicit-fallthrough with

meson.build

@@ -2198,7 +2198,6 @@ vectorize_cflags = cc.get_supported_arguments(['-ftree-vectorize'])
 unroll_loops_cflags = cc.get_supported_arguments(['-funroll-loops'])
 
 common_warning_flags = [
-  '-Wmissing-prototypes',
   '-Wpointer-arith',
   # Really don't want VLAs to be used in our dialect of C
   '-Werror=vla',
@@ -2211,7 +2210,15 @@ common_warning_flags = [
   '-Wformat-security',
 ]
 
-cflags_warn += cc.get_supported_arguments(common_warning_flags)
+# C-only warnings
+c_warning_flags = [
+  '-Wmissing-prototypes',
+  '-Wold-style-declaration',
+  '-Wold-style-definition',
+  '-Wstrict-prototypes',
+]
+
+cflags_warn += cc.get_supported_arguments(common_warning_flags, c_warning_flags)
 if have_cxx
   cxxflags_warn += cxx.get_supported_arguments(common_warning_flags)
 endif
@@ -2252,9 +2259,6 @@ if cc.has_argument('-Wmissing-variable-declarations')
   cflags_no_missing_var_decls += '-Wno-missing-variable-declarations'
 endif
 
-# These are C-only flags, supported in all C11-capable GCC/Clang versions.
-cflags_warn += cc.get_supported_arguments(['-Wstrict-prototypes', '-Wold-style-definition'])
-
 # The following tests want to suppress various unhelpful warnings by adding
 # -Wno-foo switches.  But gcc won't complain about unrecognized -Wno-foo
 # switches, so we have to test for the positive form and if that works,