As of autoconf-2.65 the AC_LANG_SOURCE source macro no longer
includes the confdef.h results when expanded. To handle this
simply explicitly include confdef.h in conftest.c. This will
cause two copies to of confdef.h to be added to the test for
earlier autoconf versions but this is not harmful.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>