move note about macOS deployment target to new platform support page

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/markdown/Compiler-properties.md

@@ -179,20 +179,8 @@ if [[#compiler.has_function]]('open_memstream', prefix : '#include <stdio.h>')
 endif
 ```
 
-Note that, on macOS programs can be compiled targeting older macOS
-versions than the one that the program is compiled on. It can't be
-assumed that the OS version that is compiled on matches the OS version
-that the binary will run on.
-
-Therefore when detecting function availability with [[compiler.has_function]], it
-is important to specify the correct header in the prefix argument.
-
-In the example above, the function `open_memstream` is detected, which
-was introduced in macOS 10.13. When the user builds on macOS 10.13,
-but targeting macOS 10.11 (`-mmacosx-version-min=10.11`), this will
-correctly report the function as missing. Without the header however,
-it would lack the necessary availability information and incorrectly
-report the function as available.
+When targeting macOS, the header in the prefix argument is also what makes the check
+respect the deployment target.  See [Platform notes](Platform-notes.md#macos-deployment-target).
 
 ## Is a macro defined?
 

docs/markdown/Platform-notes.md

@@ -6,6 +6,36 @@ short-description: Notes and known issues for specific platforms
 
 This page collects information that is specific to a platform or to a toolchain.
 
+## macOS deployment target
+
+Programs can be compiled for older macOS versions than the one they are compiled on.  The
+version of the build machine therefore does not tell which functions are available when the
+program runs.
+
+The deployment target is chosen by the compiler.  Either set the `MACOSX_DEPLOYMENT_TARGET`
+environment variable before running Meson, or pass `-mmacosx-version-min` in both the compile
+and the link arguments, for example in a native file:
+
+```ini
+[built-in options]
+c_args = ['-mmacosx-version-min=10.11']
+c_link_args = ['-mmacosx-version-min=10.11']
+```
+
+The checks that Meson runs use the same arguments, so they see the deployment target.
+
+When detecting a function with `compiler.has_function()`, specify the header that declares it
+in the `prefix` argument.  For example, `open_memstream` was introduced in macOS 10.13.  When
+building on macOS 10.13 for macOS 10.11, the following check correctly reports the function as
+missing:
+
+```meson
+cc.has_function('open_memstream', prefix : '#include <stdio.h>')
+```
+
+Without the header, the check lacks the availability information and incorrectly reports the
+function as available.
+
 ## QNX
 
 Meson can detect and use `qcc` and `q++`, the C and C++ compiler drivers shipped with the QNX