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
and glib#2147

.

Signed-off-by: Simon McVittie <smcv@collabora.com>