PATCH: fix cross-compilation detection on Blue Gene/L

[Steven G. Johnson]

As I discussed recently on the autoconf mailing list, the cross-compiler detection on the IBM Blue Gene (when you pass --host but not --build) gives a false negative, because no-op executables still run on the build machine.

Regards,
Steven G. Johnson


2009-06-08  Steven G. Johnson  <stev...@alum.mit.edu>

        Create a file in test program when detecting cross-compilation.
        * lib/autoconf/lang.m4 (_AC_LANG_IO_PROGRAM): New macro, returns
        program that creates a file.
        (_AC_COMPILER_EXEEXT,_AC_COMPILER_EXEEXT_WORKS): Call new macro
        and document why it's needed to robustly detect cross-compiling.
        * lib/autoconf/c.m4 (_AC_LANG_IO_PROGRAM): Implement new macro.
        * lib/autoconf/fortran.m4 (_AC_LANG_IO_PROGRAM): Likewise.
        * lib/autoconf/erlang.m4 (_AC_LANG_IO_PROGRAM): Likewise.

The attached patch fixes the false negative, by creating a file in the test program that is run. This should be safe because our other autoconf tests (e.g. AC_COMPUTE_INT) already assume that files can be created (via fopen/fclose) when not cross-compiling, so it's checking for a feature we rely on anyway. The patch is against git head, and 'make check' passes for me. My copyright assignment for autoconf is already on file with the FSF for many years now.

patch.out
Description: Binary data