config/libatomic: require -latomic iff atomic.c doesn't link w/o it

In absence of LTO, and dynamic libatomic, la.so ends up in the needs
section of every toolchain executable; some consider this an issue

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#12345

config/user-libatomic.m4

@@ -1,33 +1,27 @@
 dnl #
-dnl # If -latomic exists, it's needed for __atomic intrinsics.
-dnl #
-dnl # Some systems (like FreeBSD 13) don't have a libatomic at all because
-dnl # their toolchain doesn't ship it – they obviously don't need it.
-dnl #
-dnl # Others (like sufficiently ancient CentOS) have one,
-dnl # but terminally broken or unlinkable (e.g. it's a dangling symlink,
-dnl # or a linker script that points to a nonexistent file) –
-dnl # most arches affected by this don't actually need -latomic (and if they do,
-dnl # then they should have libatomic that actually exists and links,
-dnl # so don't fall into this category).
-dnl #
-dnl # Technically, we could check if the platform *actually* needs -latomic,
-dnl # or if it has native support for all the intrinsics we use,
-dnl # but it /really/ doesn't matter, and C11 recommends to always link it.
+dnl # If -latomic exists and atomic.c doesn't link without it,
+dnl # it's needed for __atomic intrinsics.
 dnl #
 AC_DEFUN([ZFS_AC_CONFIG_USER_LIBATOMIC], [
-	AC_MSG_CHECKING([whether -latomic is present])
+	AC_MSG_CHECKING([whether -latomic is required])
 
 	saved_libs="$LIBS"
 	LIBS="$LIBS -latomic"
+	LIBATOMIC_LIBS=""
 
 	AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])], [
-		LIBATOMIC_LIBS="-latomic"
+		LIBS="$saved_libs"
+		saved_cflags="$CFLAGS"
+		CFLAGS="$CFLAGS -isystem lib/libspl/include"
+		AC_LINK_IFELSE([AC_LANG_PROGRAM([#include "lib/libspl/atomic.c"], [])], [], [LIBATOMIC_LIBS="-latomic"])
+		CFLAGS="$saved_cflags"
+	])
+
+	if test -n "$LIBATOMIC_LIBS"; then
 		AC_MSG_RESULT([yes])
-	], [
-		LIBATOMIC_LIBS=""
+	else
 		AC_MSG_RESULT([no])
-	])
+	fi
 
 	LIBS="$saved_libs"
 	AC_SUBST([LIBATOMIC_LIBS])