build: build SDCC 4.6.0 with two mcs51 fixes in the tree

Released SDCC 4.5.0 and 4.6.0 miscompile some of our code in the mcs51
register allocator (SDCC bugs 4153 and 4154, fixed in trunk r17025 and
r17039). The first make now downloads the 4.6.0 sources, checks their
sha256, applies the two fixes from sdcc/ and installs the compiler into
toolchain/, named after a checksum of sdcc/, so a changed patch builds a
new one. make CC=sdcc still uses an installed compiler.

README.md

@@ -56,12 +56,17 @@ device simulator is provided, which runs entirely under Linux as a local webserv
 
 ## (0) Compiling Requirements
 
-Install the following particular build requisites (Debian 12/13), note that Ubuntu 24.04
-still has an older version of sdcc, but you will need sdcc version 4.5 for the code to compile:
+Install the following particular build requisites (Debian 12/13):
 ```
-sudo apt install make gcc sdcc xxd python-is-python3 libjson-c-dev zlib1g-dev
+sudo apt install make gcc g++ bison flex libboost-dev patch bzip2 curl xxd python-is-python3 libjson-c-dev zlib1g-dev
 ```
 
+The first `make` downloads SDCC 4.6.0, applies the fixes in `sdcc/` and builds it into
+`toolchain/`, which takes a few minutes. Later builds reuse it. The released SDCC 4.5.0
+and 4.6.0 miscompile parts of this code, see [SDCC bug #4153](https://sourceforge.net/p/sdcc/bugs/4153/)
+and [#4154](https://sourceforge.net/p/sdcc/bugs/4154/); both fixes are in SDCC trunk since
+r17039. To use an SDCC you installed yourself instead, run `make CC=sdcc ...`.
+
 <details>
 <summary>If using Docker (click to expand)</summary>