__weak_alias() doesn't exist on FreeBSD. Use __weak_reference();

__weak_alias() doesn't exist on FreeBSD.  Use __weak_reference();

Expose iconv functions as weak symbols as well as their internal
remapped #define names.  This is necessary for autoconf compatability -
on Linux it appears that #include <iconv.h> isn't a link time
prerequisite for their version that's built into glibc.

Initialize the pthread rwlock.  Note that upstream has three
separate locks.  The file-local static lock appears intentional.

I'm using this as a ports-compatible compile-time substitute for
converters/libiconv on one of my personal machines.