Build all Flex files standalone

The proposed Meson build system will need a way to ignore certain
generated files in order to coexist with the autoconf build system,
and C files generated by Flex which are #include'd into .y files make
this more difficult. In similar vein to 72b1e3a21, arrange for all Flex
C files to compile to their own .o targets.

Reviewed by Andres Freund

Discussion: https://www.postgresql.org/message-id/20220810171935.7k5zgnjwqzalzmtm%40awork3.anarazel.de
Discussion: https://www.postgresql.org/message-id/CAFBsxsF8Gc2StS3haXofshHCzqNMRXiSxvQEYGwnFsTmsdwNeg@mail.gmail.com

src/backend/utils/misc/guc-file.l

@@ -1,4 +1,4 @@
-/* -*-pgsql-c-*- */
+%top{
 /*
  * Scanner for the configuration file
  *
@@ -7,8 +7,6 @@
  * src/backend/utils/misc/guc-file.l
  */
 
-%{
-
 #include "postgres.h"
 
 #include <ctype.h>
@@ -19,8 +17,12 @@
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
 #include "storage/fd.h"
+#include <sys/stat.h>
+#include "utils/memutils.h"
+}
 
 
+%{
 /*
  * flex emits a yy_fatal_error() function that it calls in response to
  * critical errors like malloc failure, file I/O errors, and detection of