but it appears that gcc_tooldir is not defined. Is this a nickname for something else, and also where is it "normally"?
gcc_tooldir is a
make variable. You should find that within the scope of a GCC build, it has a value that is functionally equivalent to that of the
$(tooldir) make variable, but somewhat different in form. You are not meant to set it by hand, though you may of course use the
--with-sysroot configure option to choose your own directory for the target tools. Per the GCC build documentation:
When installing cross-compilers, GCC’s executables are not only installed into
bindir, that is,
exec-prefix/bin, but additionally into
target-alias/bin, if that directory exists. Typically, such tooldirs hold target-specific binutils, including assembler and linker.
(Emphasis in the original.)
The standard tooldir name is thus something of the form '/usr/x86_64-w64-mingw32', to lift one from the Glade example you presented in comments.