Add a check for the -Bsymbolic-functions linker flag

The -Bsymbolic-functions linker flag reduces the amount of PLT jumps in
a shared object, and has a side effect of preventing symbol collisions
in libraries and applications linking against two different shared
objects exposing the same symbol.

While the former is (generally) a performance win, the latter is less
rare than expected. For instance, PulseAudio started linking against
json-c a while ago; now, every project linking against PulseAudio is
leaking json-c symbols. In the GNOME platform, this means that projects
linking against PulseAudio cannot be safely linked against other
libraries depending on the GLib-based JSON parsing libraries JSON-GLib,
because of a symbol conflict. Nominally, this conflict would not be an
issue: libraries and applications do not need to depend on two different
JSON parsing libraries; the symbol leakage, though, ends up causing
either segmentation faults, or weird errors. For further reference,
please see: https://bugzilla.gnome.org/show_bug.cgi?id=703734

JSON-GLib already switched to using -Bsymbolic-functions, but it would
be safe if json-c did the same, wherever the linker flag is available.
Ignore and cleanup a few more files that automake creates.
Merge branch 'master' of https://github.com/ebassi/json-c into ebassi-master

Conflicts:
	Makefile.am
Merge branch 'ebassi-master'