Add and test config-version.cmake files, update example CMake style,
more cleanly test in Autotools install-exec-hook whether static linking
is enabled, clearer test matrix.

Whether built with CMake or the Autotools, libbson now installs CMake
config-file packages for bson-1.0 and bson-static-1.0, and pkg-config
files for the same.

Compile and link an example program with all combinations: libbson built
with CMake or the Autotools, using static or dynamic linking, and
finding libbson with the CMake find_package command or with pkg-config.

Distribute separate pkg-config files and CMake config packages for
programs that want to statically link to libbson.

Catch mistakes like CDRIVER-2071.

bson_api.3 and bson_index.3 man pages include all other pages, so
skip them.

Thanks to Petr Pisar for the patch.

Previously we had a thread per test; once the number exceeded circa 250,
test-libbson --threads failed on MIPS.