The steps to add this functionality are from the cmake tutorial:
https://cmake.org/cmake/help/latest/guide/tutorial/index.html
The include directories being set on the targets is necessary
for dependent targets. There is some redundancy with the
INCLUDE_DIRECTORIES calls alreday present, but I left it alone
to make the PR smaller and more approachable.
The project currently uses mixed cases in functions. This uses
lowercase on calls I touched, as this is what CMake documentation
uses and seems to be the preferred way.
I also started wrapping long lines, as some of the lines I touched
were over 170 characters.
I tested this on both Mac OS 10.15 Catalina and CentOS 7 (with
devtoolset-7) with CMake 3.10.3.
Will work on tests in CI next.
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.)
