Add some notes about new issues on MacOSX

Gah.  Not 100% sure, but it seems as if the GitHub CI system just rolled out
some updates for the macos runners, and they're messing with us.
OpenNURBS suddenly seems to need _DARWIN_C_SOURCE defined or we get the
following error:

In file included from /Users/runner/work/brlcad/brlcad/build_osx/include/openNURBS/opennurbs_system.h:389:
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:177:46: error: unknown type name 'malloc_type_id_t'
  177 |                         struct _malloc_zone_t *zone, size_t size, malloc_type_id_t type_id);
      |                                                                   ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:181:4: error: unknown type name 'malloc_type_id_t'
  181 |                         malloc_type_id_t type_id);
      |                         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:185:17: error: unknown type name 'malloc_type_id_t'
  185 |                         size_t size, malloc_type_id_t type_id);
      |                                      ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:189:4: error: unknown type name 'malloc_type_id_t'
  189 |                         malloc_type_id_t type_id);
      |                         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:194:4: error: unknown type name 'malloc_type_id_t'
  194 |                         malloc_type_id_t type_id);
      |                         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:334:2: error: unknown type name 'malloc_type_id_t'; did you mean 'malloc_type_kind_v0_t'?
  334 |         malloc_type_id_t type_id;
      |         ^
/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/malloc/malloc.h:237:3: note: 'malloc_type_kind_v0_t' declared here
  237 | } malloc_type_kind_v0_t;
      |   ^

I haven't managed to get a working configure-time test for the needs
_DARWIN_C_SOURCE yet, so for now just setting the define to get us passed the
build error.

Author: starseeker

TODO

@@ -18,6 +18,16 @@ tasks as they are completed.
 THESE TASKS SHOULD HAPPEN BEFORE THE NEXT RELEASE
 -------------------------------------------------
 
+* find a path forward on OSX with the following error:
+
+  install_name_tool: changing install names or rpaths can't be redone
+
+  At the moment it's looking like Xcode 16 introduced some
+  new restrictions in install_name_tool that break our previous
+  management logic for path updating.  That may mean we'll need
+  to leverage LIEF's abilities to mod Mac binaries (up until now
+  we've only used it for Linux) to get around the new
+  install_name_tool limitations.
 
 THESE TASKS SHOULD HAPPEN WITHIN TWO RELEASE ITERATIONS
 -------------------------------------------------------

include/brep/defines.h

@@ -72,6 +72,12 @@ extern "C++" {
 /* don't let opennurbs include windows.h */
 #define ON_NO_WINDOWS 1
 
+// TODO - need a compile time test that can tell whether this is required or
+// not - needed for building with Xcode 16, apparently. My first attempt at a
+// configure time test didn't work - need someone who can debug interactively
+// in OSX.  In the meantime add this as a temporary measure.
+#define _DARWIN_C_SOURCE
+
 #include "opennurbs.h"
 
 #if defined(__GNUC__) && !defined(__clang__)