[sr #110294] AC_LANG_PUSH/AC_LANG_POP malfunction inside AC_DEFUN

Update of sr #110294 (project autoconf):

                  Status:                    None => Invalid                
             Open/Closed:                    Open => Closed                 

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Follow-up Comment #2:

After a bunch of head-scratching I have figured out what's going on. This
code


    AC_LANG_PUSH([C++])
    AC_CACHE_CHECK([whether the C++ compiler has <cuchar>],
      [gl_cv_cxxheader_cuchar],
      [
       AC_COMPILE_IFELSE(
         [AC_LANG_PROGRAM(
            [[#include <cuchar>]],
            [[]])
         ],
         [gl_cv_cxxheader_cuchar=yes],
         [gl_cv_cxxheader_cuchar=no])
      ])
    AC_LANG_POP([C++])
-verbatim+

invokes AC_COMPILE_IFELSE with the current language set to C++. 
AC_COMPILE_IFELSE calls AC_LANG_COMPILER_REQUIRE, which AC_REQUIREs
AC_LANG_COMPILER(C++), which AC_REQUIREs AC_PROG_CXX.  Because of your earlier
"kind of an optional AC_PROG_CXX" code, this causes the AC_LANG machinery to
get confused about what the correct setting for $ac_compile is, for checks in
between the top of the AC_DEFUN and the AC_LANG_PUSH.

We can't support your code as-is in Autoconf 2.70.  However, I can offer you a
simple workaround.  Move all of the "optional AC_PROG_CXX" code into a new
AC_DEFUN.  Make the last line of that AC_DEFUN be


m4_provide([AC_PROG_CXX])


(right after the "Determine a good default for CXXFLAGS" if-block).

In my testing, those modifications make your sample configure script work
correctly.  They should be harmless with 2.69.

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