build: Add a symbol version to all exported symbols

The --default-symver linker option attaches a default version definition (the SONAME) to every exported symbol. It is supported since at least GNU binutils 2.22 in 2011 (older versions not tested).

With this version definition, newly-linked binaries that depend on the json-glib shared library will refer to its symbols in a versioned form, preventing their references from being resolved to a symbol of the same name exported by json-c or libjansson if those libraries appear in dependency search order before json-glib, which will usually result in a crash. This is necessary because ELF symbol resolution normally uses a single flat namespace, not a tree like Windows symbol resolution. At least one symbol (json_object_iter_next()) is exported by all three JSON libraries.

Linking with -Bsymbolic is not enough to have this effect in all cases, because -Bsymbolic only affects symbol lookup within a shared object, for example when json_from_string() calls json_parser_new(). It does not affect symbol lookup when external code calls into json-glib, for example when libedataserver calls json_parser_new().

This change will also not prevent code that depends on json-c or libjansson from finding json-glib's symbols and crashing; to prevent that, a corresponding change in json-c or libjansson would be needed.

Adding a symbol-version is a backwards-compatible change, but once added, removing or changing the symbol-version would be an incompatible change that requires a SONAME bump.

Partially resolves #33 (closed) and glib#2147 (closed).