Add z,relro and z,now to the GCC linker flags

These flags are mitigations against memory corruption bugs, and are
typically enabled by Linux distributions hardening rules.

We only use these flags with GCC, similarly to `-Bsymbolic`.

More information on relro is available here:

 - http://tk-blog.blogspot.co.uk/2009/02/relro-not-so-well-known-memory.html
 - http://mudongliang.github.io/2016/07/11/relro-a-not-so-well-known-memory-corruption-mitigation-technique.html
 - https://wiki.debian.org/Hardening#DEB_BUILD_HARDENING_RELRO_.28ld_-z_relro.29

src/meson.build

@@ -52,7 +52,7 @@ foreach h: headers
 endforeach
 
 if cc.get_id() == 'gcc'
-  common_ldflags = [ '-Wl,-Bsymbolic', ]
+  common_ldflags = [ '-Wl,-Bsymbolic', '-Wl,-z,relro', '-Wl,-z,now', ]
 else
   common_ldflags = []
 endif