diff --git a/Makefile.inc1 b/Makefile.inc1
index a7b61aa9630..596af0c4c2b 100644
--- a/Makefile.inc1
+++ b/Makefile.inc1
@@ -853,6 +853,14 @@ BFLAGS+=	-B${WORLDTMP}/usr/bin
     (defined(X_COMPILER_TYPE) && ${X_COMPILER_TYPE} == clang)
 XCFLAGS+=	-target ${TARGET_TRIPLE}
 .endif
+# An external compiler (e.g. upstream clang on Linux/macOS) only knows the
+# freebsd triple, so it predefines __FreeBSD__ but not __MidnightBSD__.  The
+# tree keys on __MidnightBSD__ everywhere, so supply it ourselves with the same
+# value our own clang derives from the triple's OS major version.
+.if defined(X_COMPILER_MIDNIGHTBSD_VERSION) && \
+    ${X_COMPILER_MIDNIGHTBSD_VERSION} == "unknown"
+XCFLAGS+=	-D__MidnightBSD__=${TARGET_TRIPLE:C/.*freebsd([0-9]+).*/\1/}
+.endif
 XCFLAGS+=	--sysroot=${WORLDTMP}
 
 .if !empty(BFLAGS)
diff --git a/sys/compat/linuxkpi/common/include/linux/kernel.h b/sys/compat/linuxkpi/common/include/linux/kernel.h
index e60191414e6..309b987e369 100644
--- a/sys/compat/linuxkpi/common/include/linux/kernel.h
+++ b/sys/compat/linuxkpi/common/include/linux/kernel.h
@@ -593,12 +593,6 @@ mult_frac(uintmax_t x, uintmax_t multiplier, uintmax_t divisor)
 	return ((q * multiplier) + ((r * multiplier) / divisor));
 }
 
-static inline int64_t
-abs64(int64_t x)
-{
-	return (x < 0 ? -x : x);
-}
-
 typedef struct linux_ratelimit {
 	struct timeval lasttime;
 	int counter;
diff --git a/sys/dev/drm2/drm_os_freebsd.h b/sys/dev/drm2/drm_os_freebsd.h
index 7f5bb22aff0..0ce0dede6d7 100644
--- a/sys/dev/drm2/drm_os_freebsd.h
+++ b/sys/dev/drm2/drm_os_freebsd.h
@@ -304,13 +304,6 @@ ilog2(unsigned long x)
 	return (flsl(x) - 1);
 }
 
-static inline int64_t
-abs64(int64_t x)
-{
-
-	return (x < 0 ? -x : x);
-}
-
 int64_t		timeval_to_ns(const struct timeval *tv);
 struct timeval	ns_to_timeval(const int64_t nsec);
 
diff --git a/sys/sys/libkern.h b/sys/sys/libkern.h
index 570caac448c..def05a6466a 100644
--- a/sys/sys/libkern.h
+++ b/sys/sys/libkern.h
@@ -114,6 +114,7 @@ static __inline off_t omin(off_t a, off_t b) { return (a < b ? a : b); }
 static __inline int abs(int a) { return (a < 0 ? -a : a); }
 static __inline long labs(long a) { return (a < 0 ? -a : a); }
 static __inline quad_t qabs(quad_t a) { return (a < 0 ? -a : a); }
+static __inline int64_t abs64(int64_t a) { return (a < 0 ? -a : a); }
 
 #ifndef RANDOM_FENESTRASX
 #define	ARC4_ENTR_NONE	0	/* Don't have entropy yet. */