build: Fix a few compilation issues with Clang 7 and -Werror

Summary:

• Do not check for main() in libminiupnpc

main() { main(); } causes "infinite recursion" compilation warning
which with -Werror fails the check.

• Remove redundant checks for MSG_* from configure.ac

It is redundant to check for the presence of MSG_NOSIGNAL macro in
configure.ac, define HAVE_MSG_NOSIGNAL and then check whether the later
is defined in the source code. Instead we can check directly whether
MSG_NOSIGNAL is defined. Same for MSG_DONTWAIT.

In addition to that, the checks we had in configure.ac produce a
compiler warning about unused variable and thus could fail if
-Werror is present and erroneously proclaim that the macros are
not available.

• ax_boost_{chrono,unit_test_framework}.m4: take changes from upstream

Apply changes to
build-aux/m4/ax_boost_chrono.m4 and
build-aux/m4/ax_boost_unit_test_framework.m4
from upstream: https://github.com/peti/autoconf-archive

This is a backport of Core PR12678

Test Plan:

make check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1869