Setup base code to expose a C API

[chaitan94]

A C-API would allow the library to be used by many other languages through C-based bindings.
Especially, it would also allow MobilityDB to depend on MEOS and use it where appropriate.

Some possibly useful references

• to get started on learning how to do this: https://www.teddy.ch/c++_library_in_c/
• auto generation of C interface: http://samanbarghi.com/blog/2016/12/06/generate-c-interface-from-c-source-code-using-clang-libtooling/
• Also we can always refer to GEOS's code as well.