CMake custom command for bison target

I'm trying to convert an old Autotools project to CMake. In one of the Makefiles, there are some rules that use lex and yacc. I found that CMake has bison_target and flex_target. However in the Makefile there's a rule with sed that edit the generated file. Here is the Makefile.am:

docdir = @SIS_DOCDIR@
AM_CPPFLAGS = -DSIS -I../include
AM_YFLAGS = -d

BUILT_SOURCES = readlib.c readlib.h readliblex.c
CLEANFILES = $(BUILT_SOURCES)

noinst_LIBRARIES = libgenlib.a
libgenlib_a_SOURCES = aoi.c com_genlib.c comb.c comb.h count.c genlib.c \
    genlib.h genlib_int.h io.c nand.c permute.c sptree.c sptree.h \
    readlib.y readliblex.l
dist_doc_DATA = genlib.doc

readlib.h: readlib.c
readlib.c readlib.h: readlib.y
    $(YACC) $(YFLAGS) $(AM_YFLAGS) $<
    sed 's/yy/GENLIB_yy/g' y.tab.c > readlib.c
    sed 's/yy/GENLIB_yy/g' y.tab.h > readlib.h
    $(RM) y.tab.c y.tab.h
readliblex.c: readliblex.l readlib.h
    $(LEX) $(LFLAGS) $(AM_LFLAGS) $<
    sed 's/yy/GENLIB_yy/g' lex.yy.c > readliblex.c
    $(RM) lex.yy.c

And this is how I translated it:

set(CMAKE_C_FLAGS -DSIS)
add_library(genlib STATIC
        aoi.c
        com_genlib.c
        comb.c
        count.c
        genlib.c
        io.c
        nand.c
        permute.c
        sptree.c

        readlib.y
        readliblex.l
        )
find_package(BISON)
find_package(FLEX)
bison_target(readlib readlib.y -d)
add_custom_command(TARGET readlib POST_BUILD
        COMMAND sed 's/yy/GENLIB_yy/g' y.tab.c > readlib.c
        COMMAND sed 's/yy/GENLIB_yy/g' y.tab.h > readlib.h
        COMMAND rm -f y.tab.c y.tab.h
        )
flex_target(readliblex readliblex.l lex.yy.c)
add_custom_command(TARGET readliblex POST_BUILD
        COMMAND sed 's/yy/GENLIB_yy/g' lex.yy.c > readliblex.c
        COMMAND rm -f lex.yy.c
        )

The problem is that CMake doesn't see readlib and readliblex as targets:

CMake Error at sis/genlib/CMakeLists.txt:28 (add_custom_command):
  No TARGET 'readlib' has been created in this directory.


CMake Error at sis/genlib/CMakeLists.txt:34 (add_custom_command):
  No TARGET 'readliblex' has been created in this directory.

Where am I wrong?

Given that bison_target didn't create a target, I ended up using a different solution:

set(CMAKE_C_FLAGS -DSIS)

find_package(BISON REQUIRED)
find_package(FLEX REQUIRED)

set(BISON_OUTPUT ${CMAKE_SOURCE_DIR}/sis/genlib/readlib.c)
if (BISON_FOUND)
    add_custom_command(
            OUTPUT ${BISON_OUTPUT}
            COMMAND ${BISON_EXECUTABLE}
            --output=${BISON_OUTPUT}
            -d
            ${CMAKE_SOURCE_DIR}/sis/genlib/readlib.y
            COMMENT "Generating readlib.c"
    )
endif ()

set(FLEX_OUTPUT ${CMAKE_SOURCE_DIR}/sis/genlib/readliblex.c)
if (FLEX_FOUND)
    add_custom_command(
            OUTPUT ${FLEX_OUTPUT}
            COMMAND ${FLEX_EXECUTABLE}
            --outfile=${FLEX_OUTPUT}
            ${CMAKE_SOURCE_DIR}/sis/genlib/readliblex.l
            COMMENT "Generating readliblex.c"
    )
endif ()

add_library(genlib STATIC
        aoi.c
        com_genlib.c
        comb.c
        count.c
        genlib.c
        io.c
        nand.c
        permute.c
        sptree.c
        ${BISON_OUTPUT}
        ${FLEX_OUTPUT}
        )
add_custom_command(TARGET genlib POST_BUILD
        COMMAND sed -i 's/yy/GENLIB_yy/g' ${CMAKE_SOURCE_DIR}/sis/genlib/readlib.c
        COMMAND sed -i 's/yy/GENLIB_yy/g' ${CMAKE_SOURCE_DIR}/sis/genlib/readlib.h
        COMMAND sed -i 's/yy/GENLIB_yy/g' ${CMAKE_SOURCE_DIR}/sis/genlib/readliblex.c
        )