Adds support for the LITE_RUNTIME optimization option to the protobuf-c
compiler. Enabling this option would generate lighter weight message,
enum, and service descriptors that contain NO strings. As a result,
calls to lookup descriptors via the *_get_{field,value,method}_by_name
API will return NULL.

Default compiler behavior (when optimize_for is not specified or is not
set to LITE_RUNTIME) is unchanged.

Certain protobuf comments could generate invalid C comments
and inadvertently close the comment block. This commit removes '/'
signs in such comments.

One example of a .proto file containing such comments is a commonly
included descriptor.proto from the protobuf library.

Check for NULL pointer for repeated fields passed to protobuf_c_message_free_unpacked

Closes #177

Check for NULL message pointer passed to protobuf_c_message_free_unpacked

Closes #177

warning: assigning to 'uint8_t *' (aka 'unsigned char *')
from 'char *' converts between pointers to integer types
with different sign [-Wpointer-sign]

Add test cases for parsing multiple fields of the same oneof from the wire
Add more tests for submessage merging, including oneofs

Add a field flag and functions to pack/unpack oneofs
Add logic to parse multiple fields for the same oneof
Add logic for merging submessages that contain oneofs

Ironically, we need to tell automake to distribute the CMakeLists.txt
file in the tarball...

For certain platforms where autotools is not the preferred build system,
provide a fallback cmake file that can compile protoc-c and a static
library of libprotobuf-c.

Based on the file from alex85k's protobuf-c repository.

(Issue #168.)