libtool 2.2.2 installable libltdl: bogus LTDLINCL?

[Thien-Thi Nguyen]

On my system, there is no /usr/include/ltdl.h, however there exists:
 /usr/lib/libltdl.so.3
 /tmp/a/b/c/include/ltdl.h
 /tmp/a/b/c/lib/libltdl.so.7

Here is a fragment of the configure.in that i use
to try to get an installable libltdl:

  LT_INIT([dlopen])
  LT_CONFIG_LTDL_DIR([libltdl])
  LTDL_INIT([installable])
  
  # blech
  AC_MSG_CHECKING([if LTDLINCL is bogus])
  if test "$ac_cv_header_ltdl_h" = no \
     && test "$ac_cv_lib_ltdl_lt_dlinit" = yes \
     && test x"$LTDLINCL" = x ; then
     LTDLINCL='-I${top_srcdir}/libltdl'
     AC_MSG_RESULT([yes, jamming to: $LTDLINCL])
  else
     AC_MSG_RESULT([no])
  fi

The lines after the blech are a required workaround
that i believe is due to the ltdl.m4 macros looking in
/usr/include for the header but ${prefix}/lib for the
shared object.

The workaround is triggered with the command:

  ./configure --prefix /tmp/a/b/c

Am i missing something in my libtool usage (or analysis)?
Semi-related: I realize that there are configure options

  --with-included-ltdl
  --with-ltdl-include
  --with-ltdl-lib

but am wondering if libtool behavior out of the box
can't be improved to not require specifying them.

thi