Add some documentation about building various software

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+# Software notes
+
+This document lists some software known to build successfully, and any
+special procedures necessary.
+
+## [sbase]
+
+Builds without issue as of [ef9f6f35].
+
+[sbase]: https://core.suckless.org/sbase/
+[ef9f6f35]: https://git.suckless.org/sbase/commit/ef9f6f359a0762b738302ae05822514d72b70450.html
+
+## [binutils]
+
+QBE must be built with `NPred` (in `all.h` at least 297).
+
+On glibc systems, you must make sure to include `crtbegin.o` and
+`crtend.o` from gcc at the end of `startfiles` and beginning of `endfiles`
+respectively.
+
+On musl systems, you must define `long double` to match `double` to
+avoid errors in unused `static inline` functions in musl's `math.h`.
+
+Requires several patches available here:
+https://github.com/michaelforney/binutils-gdb/
+
+- Fix function pointer subtraction in `bfd/doc/chew.c` (applied upstream).
+- Skip unsupported `LDFLAGS`, only tested to work against `CXX` by
+  configure, but applied to `CC` as well.
+- Disable `long double` support in `_bfd_doprnt`.
+- Alter some ifdefs to avoid statement expressions and VLAs.
+- Implement `pex_unix_exec_child` with `posix_spawn` instead of `vfork`
+  and subtle `volatile` usage.
+- Make `regcomp` and `regexec` match the header declaration in usage of
+  `restrict`.
+- Don't declare `vasprintf` unless it was checked for and not
+  found. Several subdirectories in binutils include `libiberty.h`,
+  but don't use `vasprintf`, causing conflicting declarations with libc
+  in usage of `restrict`.
+- Make sure `config.h` is included in `arlex.c` so that the appropriate
+  feature-test macros get defined to expose `strdup`.
+
+Configure with
+
+	./configure CC=/path/to/cc CFLAGS_FOR_BUILD=-D_GNU_SOURCE \
+		--disable-intl --disable-gdb --disable-plugins --disable-readline