rework header/library versioning

this replaces the changes in Issue #53 with a slightly different way of
representing / retrieving the version number.

protobuf_c_version() returns the version of the *library* as a string.

protobuf_c_version_number() returns the version of the *library* as an
integer.

PROTOBUF_C_VERSION is the version of the *headers* as a string constant.

PROTOBUF_C_VERSION_NUMBER is the version of the *headers* as an integer.
t/: add new test program 'version'
ChangeLog: versioning updates related to Issue #53