Fixes two cases that were conditional on MSVC and shouldn't have been:

* Uses WIN32 for requiring ws2_32 or not
* Uses UNIX for configuring Threads or not

The latter fix just moves `find_package(Threads)` next to commands
in a later "if UNIX" stanza that uses CMAKE_THREADS_LIBS_INIT.

This commit changes .cmake files to return libraries with absolute paths,
which removes the need for CMake end-users to set `link_directories`.

See https://cmake.org/cmake/help/v3.0/command/link_directories.html
for additional context.

add_definitions() has global effect, so isn't good to include
in a .cmake file.  This commit instead publishes the definitions in a
variable and lets consumers set it on a target the same way they set
library/include paths.

libmongoc will need a corresponding update.

CDRIVER-2132: Change cmake to require only a C compiler

When a CMake project links to libbson statically it should do:

find_package (libbson-static-1.0)

With this change, the above command now sets BSON_STATIC_LIBRARIES, not
BSON_LIBRARIES, so a CMake project can now link some of its targets to
libbson statically and some dynamically.

Also, define BSON_STATIC when building the libbson static library. This
avoids marking libbson's public functions for dynamic-library export
when building libbson's static lib. It probably has no effect either
way but seems like the logical thing to do.

Also test linking with cmake config files on Windows in Evergreen.

Re-applied after reverting our changes to jsonsl.h and jsonsl.c.
Originally #da366bb2.