Don't install the libraries directly in lib/.

Instead use a subdirectory (prefix/lib/ipsec by default). Also moved the
plugins from libexec to a subdirectory of that dir.

src/libstrongswan/Makefile.am

@@ -1,4 +1,4 @@
-lib_LTLIBRARIES = libstrongswan.la
+ipseclib_LTLIBRARIES = libstrongswan.la
 
 libstrongswan_la_SOURCES = \
 library.c library.h \
@@ -80,6 +80,7 @@ libstrongswan_la_LIBADD = $(PTHREADLIB) $(DLLIB) $(BTLIB) $(SOCKLIB) $(RTLIB)
 INCLUDES = -I$(top_srcdir)/src/libstrongswan
 AM_CFLAGS = \
 -DIPSEC_DIR=\"${ipsecdir}\" \
+-DIPSEC_LIB_DIR=\"${ipseclibdir}\" \
 -DPLUGINDIR=\"${plugindir}\" \
 -DSTRONGSWAN_CONF=\"${strongswan_conf}\"
 

src/libstrongswan/library.c

@@ -27,7 +27,7 @@
 #include <utils/leak_detective.h>
 #endif
 
-#define CHECKSUM_LIBRARY IPSEC_DIR"/libchecksum.so"
+#define CHECKSUM_LIBRARY IPSEC_LIB_DIR"/libchecksum.so"
 
 typedef struct private_library_t private_library_t;