Merge branch 'bugfix/lwip_makefile_ppp' into 'master'

lwip: fix error when building on OS X, only build ppp if enabled

- reorder directories listed in COMPONENT_SRCDIRS so that
  subdirectories precede parent directories

- don’t include PPP source directories if PPP support is not enabled

See merge request !700

components/lwip/component.mk

@@ -2,9 +2,22 @@
 # Component Makefile
 #
 
-COMPONENT_ADD_INCLUDEDIRS := include/lwip include/lwip/port include/lwip/posix apps/ping
+COMPONENT_ADD_INCLUDEDIRS := \
+	include/lwip \
+	include/lwip/port \
+	include/lwip/posix \
+	apps/ping
 
-COMPONENT_SRCDIRS := api apps/sntp apps/ping apps core/ipv4 core/ipv6 core netif netif/ppp netif/ppp/polarssl port/freertos port/netif port/debug port
+ifdef CONFIG_PPP_SUPPORT
+LWIP_PPP_DIRS := netif/ppp/polarssl netif/ppp 
+endif
+
+COMPONENT_SRCDIRS := \
+	api \
+	apps/sntp apps/ping apps \
+	core/ipv4 core/ipv6 core \
+	$(LWIP_PPP_DIRS) netif \
+	port/freertos port/netif port/debug port
 
 CFLAGS += -Wno-address  # lots of LWIP source files evaluate macros that check address of stack variables