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This only applies to cross-compilation builds. The `make links` rules don't
know a pre-determined list of subdirectories that need to be converted into
filename extensions. Instead, files that should have no filename extension
are usually simply identified by the absence of a `/` in their specification
in `SOURCES_TO_SYMLINK`. However, this is insufficient to handle some rarer
cases, such as a `Makefile` within a `Test` subdirectory.

To handle this, we introduce explicit support for specifying such files in
`SOURCES_TO_SYMLINK`, using a double slash where there would otherwise be a
filetype subdirectory name, such as `Test//Makefile`. (Conveniently, this is
also valid Posix filename syntax anyway.) The way this is achieved is to
tweak the regular expressions to require the directory and leafnames between
slashes to be at least one character long.

Version 7.58. Tagged as 'BuildSys-7_58'

`AAsmModule`, `CApp`, `CLibrary` and `CModule` implicitly include various
other shared makefile fragments to permit simpler master makefiles. However,
when cross-compiling, they were only included when invoked from within the
`objs` subdirectory, which in particular meant you couldn't rely on any of
the definitions from `StdTools` when extending the `clean` rule unless you
explicitly included it from the master makefile, thereby defeating the
purpose of the implicit inclusion. Change them so that `StdTools` is included
for all rules.

`TOKHELPSRC`, `HELPSRC` and `TOKENS` have the same meaning as in `AAsmModule`.
Because `CModule`, unlike `AAsmModule`, can build a binary from multiple
linked object files, it is also necessary to specify which object file(s)
depend on the autogenerated source file. This is achieved with the new input
variable `TOKHELPDEPENDS`. This defaults to `OBJS` which in turn defaults to
`TARGET`, which should mean that any components converted from `AAsmModule`
will not require any such line to be specified in their master makefile.

Requires RiscOS/Sources/Programmer/Debugger!4

Various components use objasm for its general macro abilities in order to
generate intermediate source or header files. To date, this has required
writing of custom rules, which then need separate implementations for
cross-compilation use.

There is no standardisation of destination directory (it may be generating
source or header files in any arbitrary language) so the solution here is to
specify each target using a subdirectory, basename and extension, and leave
CModule to deal with whether or not the extension should be in prefix or
suffix position, depending on the host OS. Only one such autogenerated file
is currently supported per makefile:

```
ASM2TXT = <basename> <optional extension>
ASM2TXT_SUBDIR = <optional subdirectory, each element followed by ${SEP}>
```

The source file passed to objasm is derived from `<basename>`, prefixed by
`s.` or suffixed by `.s` as appropriate.

Version 7.38. Tagged as 'BuildSys-7_38'

Detail:
  ModuleDB:
  * Correct capitalisation of resgen's TARGET to match its main source file
  GNUmakefiles/AAsmModule:
  * Targets now given ,ffa filetype suffix
  * Support source directory layouts s/<subdir>/<leaf> as used by the
    kernel and printer drivers - define SYMLINK_EXT_FIRST to enable
  * Prevent relinking when running make on an up-to-date component due to
    a difference between amu and GNU make's handling of double-colon rules
  * Support linking with GNU toolchain
  * Fix installation rule (mixup between MERGEDMDIR and MERGEDRDIR)
  * Support up to 16 assembler and 8 C-from-assembler headers, up from 3
    of each (here's looking at you, kernel) - long-term, it might be worth
    changing this to a scheme like that used by CModule to remove any
    limits
  * Fix C-from-assembler exports to have .h suffix
  GNUmakefiles/AppLibs:
  * Define INCLUDE_OSLIB to permit OSLib header search paths to be
    specified in an OS-agnostic way
  GNUma...