base_yylex undefined in src/interface/ecpg/preproc/parser.c

Hello,

with the newest flex (v2.6.2-19-g6bea32e, which is newer than 2.6.3) I
need this patch to compile postgres:

diff --git a/src/interfaces/ecpg/preproc/pgc.l
b/src/interfaces/ecpg/preproc/pgc.l
index 0453409d37..1c82b58e18 100644
--- a/src/interfaces/ecpg/preproc/pgc.l
+++ b/src/interfaces/ecpg/preproc/pgc.l
@@ -30,7 +30,7 @@ static int xcdepth = 0; /* depth of
nesting in slash-star comments */
static char *dolqstart = NULL; /* current $foo$ quote start
string */
static YY_BUFFER_STATE scanbufhandle;
static char *scanbuf;
-
+#define yylex base_yylex
/*
* literalbuf is used to accumulate literal values when multiple rules
* are needed to parse a single literal. Call startlit to reset buffer

otherwise the linker fails with:

gcc -Wall -Wmissing-prototypes -Wpointer-arith
-Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute
-Wformat-security -fno-strict-aliasing -fwrapv
-fexcess-precision=standard -g -O0 -pthread -D_REENTRANT -D_THREAD_SAFE
-D_POSIX_PTHREAD_SEMANTICS -DECPG_COMPILE -L../../../../src/port
-L../../../../src/common -L/usr/local/lib -L/usr/local/lib
-L/usr/local/lib -Wl,--as-needed
-Wl,-rpath,'/usr/local/lib',--enable-new-dtags preproc.o type.o ecpg.o
output.o parser.o keywords.o c_keywords.o ecpg_keywords.o kwlookup.o
../ecpglib/typename.o descriptor.o variable.o -lpgcommon -lpgport -lxslt
-lxml2 -lssl -lcrypto -lgssapi_krb5 -lz -lreadline -lrt -lcrypt -ldl -lm
-lpthread -o ecpg
parser.o: In function `filtered_base_yylex':
/git/postgresql/src/interfaces/ecpg/preproc/parser.c:68: undefined
reference to `base_yylex'
/git/postgresql/src/interfaces/ecpg/preproc/parser.c:105: undefined
reference to `base_yylex'
collect2: error: ld returned 1 exit status
make: *** [Makefile:42: ecpg] Error 1

because of ecpg/preproc/parser.c:68 (in REL9_5_STABLE)

With previous versions of flex the compilation has worked. But I do not
find in the code where is supposed yylex to be renamed to base_yylex
when parser.c is proccessed. pgc.l also does not use %option
prefix=base_yy, which would be the right way to rename yylex (apart from
#define YY_DECL).

What is the purpose to rename yylex to base_yylex?

Greetings
Дилян