The end of http://gcc.gnu.org/onlinedocs/gccint/Top-Level.html#Top-Level has a link to a separate manual that is supposed to explain how the top level build works, including building target libraries. Here's the corresponding text in sourcebuild.texi:
The build system in the top level directory, including how recursion into subdirectories works and how building runtime libraries for multilibs is handled, is documented in a separate manual, included with GNU Binutils. @xref{Top, , GNU configure and build system, configure, The GNU configure and build system}, for details. The link is to http://gcc.gnu.org/onlinedocs/configure/index.html#Top, which does not exist: "The requested URL /onlinedocs/configure/index.html was not found on this server." Where is it? Will it help me figure out how to set up a new target library for GCC? Janis