AC_REQUIRE mis-schedules AC_PROG_CXX and AC_PROG_CXXCPP

[Tim Van Holder]

There is a problem with the way autoconf 2.50 (as well as current
CVS HEAD) schedules AC_PROG_CXX and AC_PROG_CXXCPP if both are
AC_REQUIRE'd.

libtool.m4 has

AC_DEFUN([_LT_AC_LANG_CXX],
[AC_REQUIRE([AC_PROG_CXX])
AC_REQUIRE([AC_PROG_CXXCPP])
])# _LT_AC_LANG_CXX

but autoconf generates a configure script that has the expansion
of AC_PROG_CXXCPP _before_ the expansion of AC_PROG_CXX.
This results in AC_PROG_CXXCPP falling back on /lib/cpp, which
fails.  The only thing that seems to work (from a user's point of
view, so without changing autoconf) is explicitly calling
AC_PROG_CXX before calling AC_PROG_LIBTOOL.

Annoyingly, I've been unable to reproduce this problem with a
minimal configure.ac, so this may be a libtool issue rather than
an autoconf one.
I'll post more if and when I find out anything.