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.
