[m-rev.] for review: update README.macOS.md

[Julien Fischer]

For review by anyone.

------------------------------------------

Update README.macOS.md:

README.macOS.md:
    Mention that Apple Silicon machines are supported.

    Minor adjustments.

Julien.

diff --git a/README.macOS.md b/README.macOS.md
index 3bddf361b..5f5aede2b 100644
--- a/README.macOS.md
+++ b/README.macOS.md
@@ -1,24 +1,25 @@
 Mercury on macOS
 ================

-This file documents the port of Mercury to macOS / OS X / Mac OS X
+This file documents the port of Mercury to macOS.
 (i.e. the `*-apple-darwin*` configuration).

-Versions of macOS prior to 10.9 are not supported.
+Versions of macOS prior to 10.9 are not supported. Both Intel and Apple Silicon
+Macs are supported, although the latter has not been extensively tested.

 Contents
 --------

-* Mercury on Intel Macs
+* Mercury on macOS
 * Note for users of Xcode 11
 * Deployment Target Settings
 * Deep profiling on macOS

-Mercury on Intel Macs
-----------------------
+Mercury on macOS
+----------------

-Mercury should build and install "out-of-the-box" on Intel Macs using macOS
-10.9 onwards.
+Mercury should build and install "out-of-the-box" on Intel and Apple Silicon
+Macs using macOS 10.9 onwards.

 The `asm_fast*` and `reg*` grades are only available on 64-bit Intel systems
 and only when using GCC version 4.3 or later as the C compiler.
@@ -32,10 +33,9 @@ If the `asm_fast.gc` grade is not available, and
unless otherwise directed,
 then the `hlc.gc` grade will be used as the default grade for compiling
 applications.

-On x86-64 machines, the default is to install 64-bit versions of the
-executables and libraries in the Mercury system.  On machines running versions
-of macOS up to and including 10.14, users can choose to install 32-bit versions
-instead by invoking Mercury's `configure` script either with the option
+On machines running versions of macOS up to, and including, 10.14, users can
+choose to install a 32-bit version of Mercury by invoking Mercury's `configure`
+script either with the option

 ```
     --with-cc="clang -m32"
@@ -49,18 +49,18 @@ or

 depending on their preferred compiler. In either case, every invocation
 of the selected C compiler on Mercury-generated C files will specify
-the `-m32` option. The reason for the limitation to 10.14 is that
+the `-m32` option. The reason for the restriction to version 10.14 is that
 32-bit executables are not supported at all from macOS 10.15 onwards.

 If, after installing Mercury, you encounter errors about missing `.mih` files,
-and you have fink installed, then try removing the fink components from your
-`PATH` (these usually start with `/sw`) and reinstalling.
+and you have fink installed,try removing the fink components from your `PATH`
+(these usually start with `/sw`) and reinstalling.

 Note for users of Xcode 11
 --------------------------

 Versions of Xcode 11 prior to 11.4 ship with a broken version of `clang`
-that *cannot* be used to compile Mercury.  The `configure` script will print
+that *cannot* be used to compile Mercury. The `configure` script will print
 a warning if it detects that you are using the broken version.

 A discussion of this issue can be found at