The configure script for GLib appears to assume that libiconv will have libiconv_open() iff it's GNU iconv, so that USE_LIBICONV_GNU is defined in config.h iff libiconv has libiconv_open().

glib/gconvert.c assumes that _LIBICONV_H is defined iff libiconv is GNU iconv.

The libiconv in Mac OS X is GNU iconv; however, it only defines libiconv_open() in 32-bit mode.  In 64-bit mode, only iconv_open() is available, so the configure script thinks libiconv is not GNU iconv.  However, as it is GNU iconv, iconv.h defines _LIBICONV_H, and the tests

#if defined(USE_LIBICONV_GNU) && !defined (_LIBICONV_H)
#error GNU libiconv in use but included iconv.h not from libiconv  
#endif

in glib/gconvert.c fails.

Configuring with ./configure --with-libiconv=native also fails.