Drop warning-free support for Flex 2.5.35

This removes all the various workarounds for avoiding compiler
warnings with Flex 2.5.35.  Several recent patches have added
additional warnings that would either need to be fixed along the lines
of the existing workarounds, or we decide to no longer care about
this, which we do here.

Flex 2.5.35 is extremely outdated, and you can't even download it
anymore from any of the Flex project sites, so it's nearly impossible
to support.

After this, using Flex 2.5.35 will still work, but the generated code
will produce numerous compiler warnings.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/1a204ccd-7ae6-478c-a431-407b5c48ccc6@eisentraut.org

src/fe_utils/Makefile

@@ -51,7 +51,6 @@ libpgfeutils.a: $(OBJS)
 
 psqlscan.c: FLEXFLAGS = -Cfe -p -p
 psqlscan.c: FLEX_NO_BACKUP=yes
-psqlscan.c: FLEX_FIX_WARNING=yes
 
 # libpgfeutils could be useful to contrib, so install it
 install: all installdirs

src/fe_utils/meson.build

@@ -23,7 +23,7 @@ fe_utils_sources = files(
 psqlscan = custom_target('psqlscan',
   input: 'psqlscan.l',
   output: 'psqlscan.c',
-  command: [flex_cmd, '--no-backup', '--fix-warnings', '--', '-Cfe', '-p', '-p'],
+  command: [flex_cmd, '--no-backup', '--', '-Cfe', '-p', '-p'],
 )
 generated_sources += psqlscan
 fe_utils_sources += psqlscan

src/fe_utils/psqlscan.l

@@ -61,15 +61,6 @@ typedef int YYSTYPE;
 
 #define ECHO psqlscan_emit(cur_state, yytext, yyleng)
 
-/*
- * Work around a bug in flex 2.5.35: it emits a couple of functions that
- * it forgets to emit declarations for.  Since we use -Wmissing-prototypes,
- * this would cause warnings.  Providing our own declarations should be
- * harmless even when the bug gets fixed.
- */
-extern int	psql_yyget_column(yyscan_t yyscanner);
-extern void psql_yyset_column(int column_no, yyscan_t yyscanner);
-
 %}
 
 %option reentrant