add config/install libbson-1.0.pc to cmake build

Added missing librt to cmake which is required by clock_gettime.

librt does not automatically link for Ubuntu 12.04 for example,
so I added some changed to explicily link against it when found.
The changes to the CMakeLists.txt should be cross platform safe
since the changes are all conditionalized with if (RT_LIBRARY)

added missing bson-json.h to install list

in the read callback for bson_json_reader_read, if the callback returned
-1, we did some meaningless work and wrote a nullbyte past the beginning
of an array.

Fix that up

C++ inclusion fixes.

MSVC C++ has ssize_t and size_t types already. There is no need to define them

sometimes compilers/libraries contain max/min macros (for C++)
this may caase unclear error messages.

ignore trailing whitespace in bson_json_reader_read

This reverts commit 875199b4.

This reverts commit 70154643.