fix mingw autoconf build

presence of "-no-undefined" seems to require dependent libraries to be
linked in.

as this file is autogenerated, someone needs to fix
"model/build-autoconf.gsl"
remove unused variable

when compiling with mingw, -Werror causes compilation to fail
fix size_t format specifier

%zd and %zu are C99, so this will still be problematic when compiling
files as C++ using Visual Studio.