else
        AC_MSG_CHECKING(if TK library is installed)
        TK=`pkg-config --silence-errors --libs tk`
        if test -z "$TK"; then
            AC_MSG_RESULT(not found.)
            TK="false"
        else
            AC_MSG_RESULT(found.)
            AC_SEARCH_LIBS(Tk_Init, [tk tk8.6], [TK="true"], [TK="false"], [])
        fi
    fi
fi

# No GTK3 nor TK found? Then verify presence of FLTK library
if test "$FLTK" = true -o "$GTK3" = false -a "$TK" = false -a "$LEGACY" = false; then

        AC_MSG_WARN(neither 'lex' nor 'flex' was found on this system - BaCon binary cannot be optimized.)
    fi
fi

# Check for 'indent'
AC_CHECK_PROG(INDENT,indent,yes,no)
if test "$INDENT" = no; then
    AC_MSG_WARN('indent' was found on this system - generated sources cannot be indented when preserved.)
fi

# Make sure CFLAGS always works in the Makefile
if test -z "CFLAGS"; then
    CFLAGS="-g -O2"
fi