CI: Import recent build matrix improvements. [skip appveyor]

CI: Import recent build matrix improvements. [skip appveyor]

Start using build_common.sh, make default assignments early and
uniformly, use Solaris-compatible command substitution and exports,
install bc on linux-amd64, deduplicate and reformat some long commands.
Run "make -s" without CFLAGS=-Werror when warnings are expected.

This implements initial support for AIX and Solaris in the tcpdump build
matrix scripts.

* AIX 7.1
M4='/opt/freeware/bin/m4' \
MATRIX_CC='gcc' \
MATRIX_CMAKE=no \
MATRIX_BUILD_LIBPCAP=yes \
./build_matrix.sh

* Solaris 9
MATRIX_CC=gcc \
MATRIX_CMAKE=no \
MATRIX_BUILD_LIBPCAP=no \
./build_matrix.sh

* Solaris 10 and 11
MATRIX_CC='gcc' \
MATRIX_CMAKE=no \
./build_matrix.sh