For musl libc instead of including <linux/if_ether.h> or <net/ethernet.h>
just define the required constant ETH_ALEN locally.

--- libnm-core/nm-utils.h	2016-04-01 23:27:51.000000000 +0200
+++ libnm-core/nm-utils.h	2016-11-01 02:37:51.933498880 +0100
@@ -29,8 +29,14 @@
 
 #include <netinet/in.h>
 
-/* For ETH_ALEN and INFINIBAND_ALEN */
+#if defined(__GLIBC__)
+/* For ETH_ALEN */
 #include <linux/if_ether.h>
+#else
+#define ETH_ALEN	6		/* Octets in one ethernet addr	 */
+#endif
+
+/* For INFINIBAND_ALEN */
 #include <linux/if_infiniband.h>
 
 #include "nm-core-enum-types.h"
--- src/platform/wifi/wifi-utils.h
+++ src/platform/wifi/wifi-utils.h.new
@@ -22,10 +22,14 @@
 #ifndef __WIFI_UTILS_H__
 #define __WIFI_UTILS_H__
 
+#if defined(__GLIBC__)
 #include <net/ethernet.h>
+#else
+#define ETH_ALEN	6		/* Octets in one ethernet addr	 */
+#endif
 #include <glib.h>
 
 #include "nm-dbus-interface.h"
 
 typedef struct WifiData WifiData;

--- src/settings/nm-settings-connection.h
+++ src/settings/nm-settings-connection.h.new
@@ -22,10 +22,14 @@
 #ifndef __NETWORKMANAGER_SETTINGS_CONNECTION_H__
 #define __NETWORKMANAGER_SETTINGS_CONNECTION_H__
 
+#if defined(__GLIBC__)
 #include <net/ethernet.h>
+#else
+#define ETH_ALEN	6		/* Octets in one ethernet addr	 */
+#endif
 
 #include <nm-connection.h>
 #include "nm-types.h"
 
 G_BEGIN_DECLS