CMake: Updates

Issue #294: "apple silicon build problem(s?)": If the "--flavor/-f" flag
is not specified on the command line, then complain loudly, print help
and exit. The script used to default to "Unix Makefiles".

Updates:

- Add missing "-DHAVE_LIBPNG" compiler command line define when the PNG
  library is detected/present for screen capture support.

- Add "clang64" to the list of MinGW64 platforms for which the
  .travis/deps.sh script can install build dependencies.

- Add PThread4W_FOUND to the condition that sets async I/O for Win32
  when using vcpkg for build dependencies.

- Add vs2022-x64, vs2019-x64 and vs2017-x64 build environments to
  build 64-bit Windows executables.

- Use simulator AIO only where needed by the simulator (i.e., the
  simulator calls/uses AIO_CHECK_EVENT in sim_instr())

  - Add "USES_AIO" flag to add_simulator() to mark a simulator that
    acutally uses asynchronous I/O.

  - Build "_aio" SIMH core library variants that have AIO turned on,
    link with the "_aio" variant when a simulator sets USES_AIO.

  - Emit a warning message when WITH_ASYNC is False (CMake configuration
    option) to notify the user/developer that some functionality will be
    crippled.

  Affected simulator builds: 3b2 family, PDP-6, PDP-11, VAX family,
  IMLAC and TT2500. The makefile and cmake/generate.py also updated
  to remain in sync with CMake.

  N.B.: Simulators still link with the underlying platform's threading
  library. SEL32 requires pthreads or equivalent threading library,
  independent of AIO.

- cmake/cmake-builder.sh

  - New "--no-aio" flag: Build simulators without async I/O.

  - New "--no-aio-intrinsics" flag: Don't build async I/O using compiler
    compare-exchange, atomic load intrinsics.

- cmake/cmake-builder.ps1

  - New "-noaio" flag: Build simulators without async I/O.

  - New "-noaiointrinsics" flag: Don't build async I/O using compiler
    compare-exchange, atomic load intrinsics.

CMake 3.28.1 INTERFACE_LINK_LIBRARIES behavior change: The file name
must now be an absolute path. Relative paths no longer accepted.
Internally, SIMH enforces this if CMAKE_VERSION >= 3.19, when REAL_PATH
was first implemented.

README-CMake.md

@@ -1189,13 +1189,15 @@ add_simulator(3b2
       convention `[sim]_test.ini` -- the argument to the `TEST` parameter is the
       `[sim]` portion of the test script's name.
 
-- Option keywords: These determine which of [six (6) simulator core libraries](#simulator-core-libraries) is
+- Option keywords: These determine which of [simulator core libraries](#simulator-core-libraries) is
   linked with the simulator.
 
   - `FEATURE_INT64`: 64-bit integers, 32-bit pointers
   - `FEATURE_FULL64`: 64-bit integers, 64-bit pointers
   - `FEATURE_VIDEO`: Simulator video support.
   - `FEATURE_DISPLAY`: Video display support.
+  - `USES_AIO`: Asynchronous I/O support (primarily useful for simulator
+    network devices.)
 
 - `PKG_FAMILY` option: This option adds the simulator to a package "family" or
   simulator packaging group, e.g., "DEC PDP simulators". The default package
@@ -1213,14 +1215,31 @@ The `CMake` build infrastructure avoids repeatedly compiling the simulator
 libraries that represents the combination of required features: 32/64 bit
 support and video:
 
-| Library          | Video | Integer size | Address size | `add_simulator` flags |
-| :--------------- | :---: | -----------: | -----------: | :-------------------- |
-| simhcore.a       | N     | 32           | 32           |                       |
-| simhi64.a        | N     | 64           | 32           | `FEATURE_INT64`       |
-| simhz64.a        | N     | 64           | 64           | `FEATURE_FULL64`      |
-| simhcore_video.a | Y     | 32           | 32           | `FEATURE_VIDEO`       |
-| simhi64_video.a  | Y     | 64           | 32           | `FEATURE_INT64`, `FEATURE_VIDEO` |
-| simhz64_video.a  | Y     | 64           | 64           | `FEATURE_FULL64`, `FEATURE_VIDEO` |
+| Library           | Video | Integer size | Address size | `add_simulator` flags |
+| :---------------- | :---: | -----------: | -----------: | :-------------------- |
+| simhcore.a        | N     | 32           | 32           |                       |
+| simhi64.a         | N     | 64           | 32           | `FEATURE_INT64`       |
+| simhz64.a         | N     | 64           | 64           | `FEATURE_FULL64`      |
+| simhcore\_video.a | Y     | 32           | 32           | `FEATURE_VIDEO`       |
+| simhi64\_video.a  | Y     | 64           | 32           | `FEATURE_INT64`, `FEATURE_VIDEO` |
+| simhz64\_video.a  | Y     | 64           | 64           | `FEATURE_FULL64`, `FEATURE_VIDEO` |
+
+In addition to these six libraries, there are six asynchronous I/O (AIO)
+variants that are built and linked into a simulator when the `USES_AIO` feature
+flag is present in `add_simulator()`'s arguments:
+
+| Library variant        | Description |
+| :--------------------- | :---------: |
+| simhcore\_aio.a        | simhcore.a with AIO support.        |
+| simhi64\_aio.a         | simhi64.a with AIO support.         |
+| simhz64\_aio.a         | simhz64.a with AIO support.         |
+| simhcore\_video\_aio.a | simhcore\_video.a with AIO support. |
+| simhi64\_video\_aio.a  | simhi64\_video.a with AIO support.  |
+| simhz64\_video\_aio.a  | simhz64\_video.a with AIO support.  |
+
+The `EXCLUDE_FROM_ALL` property is set on each of theses libraries in CMake to
+avoid building the entire matrix. Practically speaking, 10 out of the 12 total
+libraries actually build for the entire simulator suite.
 
 Internally, these core libraries are [`CMake` interface libraries][cmake_interface_library] -- when they
 are added to a simulator's executable via `target_link_libraries`, the simulator
@@ -1265,6 +1284,10 @@ add_simulator(simulator_name
     ## in conjunction with FEATURE_VIDEO
     FEATURE_DISPLAY
 
+    ## Simulator uses asynchronous I/O, i.e., calls AIO_CHECK_EVENT
+    ## in its sim_instr() instruction simulation loop:
+    USES_AIO
+
     ## Packaging "family" (group) to which the simulator belongs,
     ## for packagers that support grouping (Windows: NSIS .exe,
     ## WIX .msi; macOS)