Add installation documentation and notes on ANSI C and POSIX

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24173)

NOTES-POSIX.md

@@ -0,0 +1,20 @@
+Notes on POSIX
+==============
+
+There are few instances where OpenSSL requires a POSIX C library, at least
+version 1-2008, or compatible enough functionality.
+
+There are exceptions, though, for platforms that do not have a POSIX
+library, or where there are quirks that need working around.  A notable
+platform is Windows, where POSIX functionality may be available, but where
+the function names are prefixed with an underscore, and where some POSIX
+types are not present (such as `ssize_t`).
+
+Platforms that do have a POSIX library may still not have them accessible
+unless the following macros are defined:
+
+    _POSIX_C_SOURCE=200809L
+    _XOPEN_SOURCE=1
+
+This is, for example, the case when building with gcc or clang and using the
+flag `-ansi`.