Disable clang 16's -Wcast-function-type-strict.

This is a back-patch of commit 101c37cd into REL_14_STABLE and
REL_15_STABLE.  Those branches had commit de8feb1f3, which turned on
-Wcast-function-type, but did not disable -Wcast-function-type-strict.
This silences warnings about function pointer types without prototypes
based on new C23 rules, that we want to suppress for now.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CA%2BhUKGKnBvdrbH2LW%2B7-Lv599t9JFOHjx%3Dxw-VQmdoj%3D9585CQ%40mail.gmail.com
Discussion: https://postgr.es/m/CA%2BhUKGJvX%2BL3aMN84ksT-cGy08VHErRNip3nV-WmTx7f6Pqhyw%40mail.gmail.com

configure

@@ -6533,6 +6533,50 @@ fi
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-stringop-truncation"
   fi
+  # Suppress clang 16's strict warnings about function casts
+  NOT_THE_CFLAGS=""
+  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wcast-function-type-strict, for NOT_THE_CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wcast-function-type-strict, for NOT_THE_CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wcast_function_type_strict+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${NOT_THE_CFLAGS} -Wcast-function-type-strict"
+ac_save_c_werror_flag=$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__Wcast_function_type_strict=yes
+else
+  pgac_cv_prog_CC_cflags__Wcast_function_type_strict=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: $pgac_cv_prog_CC_cflags__Wcast_function_type_strict" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wcast_function_type_strict" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wcast_function_type_strict" = x"yes"; then
+  NOT_THE_CFLAGS="${NOT_THE_CFLAGS} -Wcast-function-type-strict"
+fi
+
+
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-cast-function-type-strict"
+  fi
 elif test "$ICC" = yes; then
   # Intel's compiler has a bug/misoptimization in checking for
   # division by NAN (NaN == 0), -mp1 fixes it, so add it to the CFLAGS.

configure.ac

@@ -563,6 +563,12 @@ if test "$GCC" = yes -a "$ICC" = no; then
   if test -n "$NOT_THE_CFLAGS"; then
     CFLAGS="$CFLAGS -Wno-stringop-truncation"
   fi
+  # Suppress clang 16's strict warnings about function casts
+  NOT_THE_CFLAGS=""
+  PGAC_PROG_CC_VAR_OPT(NOT_THE_CFLAGS, [-Wcast-function-type-strict])
+  if test -n "$NOT_THE_CFLAGS"; then
+    CFLAGS="$CFLAGS -Wno-cast-function-type-strict"
+  fi
 elif test "$ICC" = yes; then
   # Intel's compiler has a bug/misoptimization in checking for
   # division by NAN (NaN == 0), -mp1 fixes it, so add it to the CFLAGS.