Skip ACX_PTHREAD tests on macOS.

macOS always has these available without setting -pthread, and ACX_PTHREAD
does not work properly on macOS at the moment.

configure

@@ -19704,6 +19704,10 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
+case "$host" in
+*-darwin*)
+	;;
+*)
 ac_ext=c
@@ -20115,6 +20119,8 @@ ac_link='$CC -o conftest$ac_exeext $CFLAGS $CPPFLAGS $LDFLAGS conftest.$ac_ext $
 ac_compiler_gnu=$ac_cv_c_compiler_gnu
+	;;
+esac
 LDFLAGS="$LDFLAGS "
 CFLAGS="$CFLAGS $PTHREAD_CFLAGS"

configure.ac

@@ -181,7 +181,15 @@ AC_CHECK_FUNCS( \
 
 AC_CHECK_DECLS([hstrerror, strsignal, sys_siglist])
 
-ACX_PTHREAD([], AC_MSG_ERROR([Error: Cannot figure out how to use pthreads!]))
+dnl Skip pthread checks on macOS as support is always enabled,
+dnl and ACX_PTHREAD will inject flags that will throw spurious warnings.
+case "$host" in
+*-darwin*)
+	;;
+*)
+	ACX_PTHREAD([], AC_MSG_ERROR([Error: Cannot figure out how to use pthreads!]))
+	;;
+esac
 
 LDFLAGS="$LDFLAGS "
 CFLAGS="$CFLAGS $PTHREAD_CFLAGS"