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Autoconf/Automake are at pains to support ancient C compilers that didn't understand the simultaneous use of the -c and -o options (create an object file with this name). There's AM_PROG_CC_C_O and a special wrapper script, and the Automake manual warns you to use them if you want to use subdir-objects mode.
There isn't an AM_PROG_CXX_C_O. It is not hard to modify AM_PROG_CC_C_O to test the C++ compiler instead, but I wonder if it's necessary. Was there ever a Unix C++ compiler (Cfront, maybe?) that didn't support simultaneous use of -c and -o? Come to that, just how old are the C compilers that don't support it -- was there ever a C89-supporting compiler with this problem, for instance?