compile: avoid AC_PROG_CC messy rewrite

compile: avoid AC_PROG_CC messy rewrite

Instead, add an hook to AC_OUTPUT to have AM_PROG_CC_C_O invoked
automatically.

See also the long-winded discussion about automake bug#13378.

* m4/minuso.m4 (AM_PROG_CC_C_O): Bring back the old implementation,
from commit v1.13.1-55-g1ab8fb6.
* m4/init.m4 (AC_PROG_CC): Remove this horrible, hacky re-write.
* (AM_INIT_AUTOMAKE): Arrange for AM_PROG_CC_C_O to be called if
necessary.
* t/am-prog-cc-c-o.sh: Adjust to avoid spurious failure.
* t/subobj.sh: Likewise.

Suggested-by: Nick Bowler <nbowler@elliptictech.com>
Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>