Fix CI on FreeBSD 15

A recent change has replaced an unused preprocessor symbol: P_UNUSED3

freebsd/freebsd-src@33be163

Another recent change has changed the size of a spare field: mc_spare

freebsd/freebsd-src@eea3e4d

(backport <#4488>)
(cherry picked from commit 46b90a8)

Author: asomers

libc-test/build.rs

@@ -2641,7 +2641,7 @@ fn test_freebsd(target: &str) {
             "TDF_CANSWAP" | "TDF_SWAPINREQ" => true,
 
             // Unaccessible in FreeBSD 15
-            "TDI_SWAPPED" | "P_SWAPPINGOUT" | "P_SWAPPINGIN" => true,
+            "TDI_SWAPPED" | "P_SWAPPINGOUT" | "P_SWAPPINGIN" | "P_UNUSED3" => true,
 
             // Removed in FreeBSD 14 (git a6b55ee6be1)
             "IFF_KNOWSEPOCH" => true,
@@ -3062,6 +3062,8 @@ fn test_freebsd(target: &str) {
             // `tcp_snd_wscale` and `tcp_rcv_wscale` are bitfields
             ("tcp_info", "tcp_snd_wscale") => true,
             ("tcp_info", "tcp_rcv_wscale") => true,
+            // mc_spare can change in size between OS releases.  It's a spare field, after all.
+            ("__mcontext", "mc_spare") => true,
 
             _ => false,
         }

src/unix/bsd/freebsdlike/freebsd/mod.rs

@@ -4217,7 +4217,9 @@ pub const TDI_IWAIT: c_int = 0x0010;
 pub const P_ADVLOCK: c_int = 0x00000001;
 pub const P_CONTROLT: c_int = 0x00000002;
 pub const P_KPROC: c_int = 0x00000004;
+#[deprecated(since = "1.0", note = "Replaced in FreeBSD 15 by P_IDLEPROC")]
 pub const P_UNUSED3: c_int = 0x00000008;
+pub const P_IDLEPROC: c_int = 0x00000008;
 pub const P_PPWAIT: c_int = 0x00000010;
 pub const P_PROFIL: c_int = 0x00000020;
 pub const P_STOPPROF: c_int = 0x00000040;