Use libdir as the install prefix for plugins

Use the libdir variable from config.mk just like lib/ uses to ensure
that the plugins are installed into the same location as the library.
This fixes systems that use lib64 for libraries like most 64 bit
Linux distributions.

Signed-off-by: Peter Robinson <pbrobinson@gmail.com>

plugins/dynamic-security/Makefile

@@ -77,10 +77,10 @@ test:
 install: all
 ifeq ($(WITH_CJSON),yes)
 ifeq ($(WITH_TLS),yes)
-	$(INSTALL) -d "${DESTDIR}$(prefix)/lib"
+	$(INSTALL) -d "${DESTDIR}$(libdir)"
-	$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"
 endif
 endif
 
 uninstall :
-	-rm -f "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	-rm -f "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"

plugins/message-timestamp/Makefile

@@ -20,8 +20,8 @@ test:
 
 install: ${PLUGIN_NAME}.so
 	# Don't install, these are examples only.
-	#$(INSTALL) -d "${DESTDIR}$(prefix)/lib"
+	#$(INSTALL) -d "${DESTDIR}$(libdir)"
-	#$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	#$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"
 
 uninstall :
-	-rm -f "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	-rm -f "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"

plugins/payload-modification/Makefile

@@ -20,8 +20,8 @@ test:
 
 install: ${PLUGIN_NAME}.so
 	# Don't install, these are examples only.
-	#$(INSTALL) -d "${DESTDIR}$(prefix)/lib"
+	#$(INSTALL) -d "${DESTDIR}$(libdir)"
-	#$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	#$(INSTALL) ${STRIP_OPTS} ${PLUGIN_NAME}.so "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"
 
 uninstall :
-	-rm -f "${DESTDIR}${prefix}/lib/${PLUGIN_NAME}.so"
+	-rm -f "${DESTDIR}${libdir}/${PLUGIN_NAME}.so"