exact version coupling between the compiler and the public headers is
too strict because some existing projects (such as collectd,
riemann-c-client, and nmsg) directly embed .pb-c.h files generated by
protoc-c into their exported headers. this would cause unnecessary build
failures in downstream clients of these libraries if a newer version of
the protobuf-c headers is installed.
however, it's still desireable to be able to explicitly declare when
compatibility is broken between the headers and the compiler, so
introduce new variables that allow independently setting the minimum
header version required by the compiler and the minimum compiler version
required by the header.
this follows the protobuf C++ implementation a little bit more closely,
though we don't have an analogous facility for verifying that the header
and *library* are compatible. (this seems like overkill for a C project;
in practice the headers and the library will be from the same version,
especially in downstream distributors like debian where the -dev package
has an exact versioned dependency on the shared library package.)
this adds a version guard like the protobuf C++ implementation. it
ensures that protoc-c and <protobuf-c.h> are from the exact same version
of protobuf-c.
per https://code.google.com/p/protobuf/source/detail?r=50, the license
on google-originated protobuf code was switched from Apache-2.0 to
BSD-3-Clause (so-called "New BSD" license). update any of the google
license statements to use this new license.
per email with dave, drop the third clause on his BSD-3-Clause license,
so this now becomes BSD-2-Clause.
make sure to use consistent indentation and wrapping throughout.
