<sys/cdefs.h>: For C++, unhide our interfaces via __{BEGIN,END}_DECLS.

<sys/cdefs.h>: For C++, unhide our interfaces via __{BEGIN,END}_DECLS.

This follows up on a previous (pthread specific) commit by alexh
(e7322b09faff75b1298e9bfe444633425d173536) that used a per-proto
macro. Revert this commit, too. In principle, we do it like NetBSD
now.

See http://gcc.gnu.org/wiki/Visibility in the GCC wiki for details
on the issue.

This solution was suggested by Martin Husemann in a thread about
xulrunner building issues:

http://mail-index.netbsd.org/tech-pkg/2012/03/11/msg008680.html

Reported-and-tested-by: marino and others