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/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;