tests on warnings: fix post-merge spurious failure

tests on warnings: fix post-merge spurious failure

* t/warnopts.sh.sh: Adjust to reflect the fact that subdir sources no
longer require AM_PROG_CC_C_O in 'configure.ac'.  Instead, use the
fact that the 'aux' directory name is diagnosed as being reserved on
Windows.
* t/warnings-override.sh: Likewise.
* t/warnings-precedence.sh: Likewise.
* t/warnings-strictness-interactions.sh: Likewise.
* t/warnings-win-over-strictness.sh: Likewise.
* t/warning-groups-win-over-strictness.sh: Adjust to reflect the fact
that subdir sources no longer require AM_PROG_CC_C_O in 'configure.ac'.
Instead, use the fact that AUTOMAKE_OPTIONS cannot hold conditional
content.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>