Define a RSVG_API macro which is defined to nothing by default so that
we can override it later in the build files to export the public and
semi-public symbols, in order that we can drop the rsvg.symbols file
that is currently used for Windows/MSVC builds.

Also define RSVG_DEPRECATED and RSVG_DEPRECATED_FOR to make use of
RSVG_API, too, so that symbols decorated with those can automatically
gain support for exporting via overriding RSVG_API.