Extracted some parts from Android.mk.in which can be used for NDK builds.

.gitignore

@@ -1,4 +1,4 @@
-/Android.mk
+/Android.common.mk
 Makefile
 Makefile.in
 aclocal.m4

Android.common.mk.in

@@ -0,0 +1,19 @@
+# some common definitions used by the main and the NDK-specific Android.mk
+# include this after strongswan_PLUGINS has been defined
+
+# helper macros to only add source files for plugins included in the list above
+# source files are relative to the android.mk that called the macro
+plugin_enabled = $(filter $(1), $(strongswan_PLUGINS))
+add_plugin = $(if $(call plugin_enabled,$(1)), \
+               $(patsubst $(LOCAL_PATH)/%,%, \
+                 $(wildcard \
+                   $(subst %,$(subst -,_,$(strip $(1))), \
+                     $(LOCAL_PATH)/plugins/%/%*.c \
+                    ) \
+                  ) \
+                ) \
+              )
+
+# strongSwan version, replaced by top Makefile
+strongswan_VERSION := "@PACKAGE_VERSION@"
+

Android.mk

@@ -32,18 +32,7 @@ strongswan_PLUGINS := $(sort $(strongswan_CHARON_PLUGINS) \
 			     $(strongswan_STARTER_PLUGINS) \
 			     $(strongswan_SCEPCLIENT_PLUGINS))
 
-# helper macros to only add source files for plugins included in the list above
-# source files are relative to the android.mk that called the macro
-plugin_enabled = $(filter $(1), $(strongswan_PLUGINS))
-add_plugin = $(if $(call plugin_enabled,$(1)), \
-               $(patsubst $(LOCAL_PATH)/%,%, \
-                 $(wildcard \
-                   $(subst %,$(subst -,_,$(strip $(1))), \
-                     $(LOCAL_PATH)/plugins/%/%*.c \
-                    ) \
-                  ) \
-                ) \
-              )
+include $(LOCAL_PATH)/Android.common.mk
 
 # includes
 strongswan_PATH := $(LOCAL_PATH)
@@ -53,7 +42,6 @@ libgmp_PATH := external/strongswan-support/gmp
 openssl_PATH := external/openssl/include
 
 # some definitions
-strongswan_VERSION := "@PACKAGE_VERSION@"
 strongswan_DIR := "/system/bin"
 strongswan_SBINDIR := "/system/bin"
 strongswan_PIDDIR := "/data/misc/vpn"

Makefile.am

@@ -6,17 +6,17 @@ endif
 
 ACLOCAL_AMFLAGS = -I m4/config
 
-EXTRA_DIST = Doxyfile.in LICENSE Android.mk.in Android.mk
+EXTRA_DIST = Doxyfile.in LICENSE Android.common.mk.in Android.common.mk Android.mk
 CLEANFILES = Doxyfile
-BUILT_SOURCES = Android.mk
-MAINTAINERCLEANFILES = Android.mk
+BUILT_SOURCES = Android.common.mk
+MAINTAINERCLEANFILES = Android.common.mk
 
 if USE_DEV_HEADERS
 config_includedir = $(ipseclibdir)/include
 nodist_config_include_HEADERS = config.h
 endif
 
-Android.mk :	Android.mk.in configure.in
+Android.common.mk :	Android.common.mk.in configure.in
 		sed \
 		-e "s:\@PACKAGE_VERSION\@:$(PACKAGE_VERSION):" \
 		$(srcdir)/$@.in > $@