Merge branch 'msvc' into master

Merge branch 'msvc' into master

Commits merged from msvc:
 - tests: few improvements to some `ar-lib' related tests
 - warnings: new 'extra-portability' category, for AM_PROG_AR
 - ar-lib: new 'AM_PROG_AR' macro, triggering the 'ar-lib' script

Extra changes required by the testsuite:
* tests/instspc-tests.sh: Adjust to new portability requirements
due to the new AM_PROG_AR macro.
* tests/libobj-basic.test: Likewise.
* tests/libobj2.test: Likewise.
* tests/libobj15a.test: Likewise.
* tests/libobj15b.test: Likewise.
* tests/libobj15c.test: Likewise.
* tests/libobj16a.test: Likewise.
* tests/libobj16b.test: Likewise.
* tests/libobj17.test: Likewise.
* tests/libobj18.test: Likewise.
* tests/libobj19.test: Likewise.
* tests/libobj20a.test: Likewise.
* tests/libobj20b.test: Likewise.
* tests/libobj20c.test: Likewise.
* tests/canon6.test: Likewise.
* tests/canon6.test: Likewise.
* tests/canon7.tests: Likewise.
* tests/extra9.test: Likewise.
* tests/extradep.test: Likewise.
* tests/extradep2.test: Likewise.
* tests/posixsubst-ldadd.test: Likewise.
* tests/posixsubst-libraries.test: Likewise.
* tests/posixsubst-ltlibraries.test: Likewise.
* tests/python-virtualenv.test: Likewise.
* tests/vartypos.test: Likewise.
* tests/vartypo2.test: Likewise.
* tests/suffix.test: Update to take into account previous
master-only changes.
* tests/suffix2.test: Likewise.
* tests/libobj7.test: Call automake with the `--add-missing'
option, instead of creating a dummy `ar-lib' file, since this
test now also runs "./configure" and "make".
* tests/suffix5.test: Copy the real `ar-lib' script file,
instead of creating a dummy one, since this test now also
runs "./configure" and "make".
* tests/extra-portability2.test: Fix this test not to rely
on the older, faulty semantics of "strictness specification
always reset warning level", which has been fixed in commit
v1.11-623-g1609491 (see also automake bug#7669 a.k.a. PR/547).
Since we are at it, throw inf few other minor unrelated
improvements.
* tests/extra-portability.test: Explicitly pass `-Wall' to
automake calls, for clarity.