build: Add a symbol version to all exported symbols for glibc

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
jansson 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 json-glib if those libraries appear in
dependency search order before jansson, 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 parse_json() calls json_decref(). It does not affect
calls from external code into jansson, unless jansson was statically
linked into the external caller.

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

Resolves: https://github.com/akheron/jansson/issues/523
(when combined with an equivalent change to json-c).

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

src/Makefile.am

@@ -26,4 +26,5 @@ libjansson_la_LDFLAGS = \
 	-no-undefined \
 	-export-symbols-regex '^json_|^jansson_' \
 	-version-info 17:0:13 \
+	@JSON_SYMVER_LDFLAGS@ \
 	@JSON_BSYMBOLIC_LDFLAGS@