The dockerfiles install both tools, and the scripts that use them
(build-make.sh and build-libssp.sh) can make do with either curl
or wget, but e.g. macOS ships with curl but not wget.
This simplifies build-mingw-w64.sh. When building on native
windows (where we also provide unprefixed tools, we can't have
the yet incomplete compiler toolchain in $PATH when building
the mingw-w64 tools. (Logically, build-mingw-w64-tools belong
with build-llvm and install-wrappers; using the original system
compiler to build tools that are supposed to run on the current
machine, as opposed to the cross target, like when building the
runtimes.)
gcc/libssp's configure script runs checks for flags that clang doesn't
implement. We actually just need to set a few HAVE defines and compile
the .c sources, so just check out this subdirectory using svn and build
it using a custom makefile.