Store and use NXT_LIBRT for go module.

auto/modules/go

@@ -108,7 +108,7 @@ ${NXT_GO}:
 	install -p ./src/nginext/*.c ./src/nginext/*.h \\
 		./src/nginext/*.go \$(GOPATH)/src/nginext/
 	CGO_CFLAGS="-I\$(NXT_ROOT)/build -I\$(NXT_ROOT)/src" \\
-		CGO_LDFLAGS="-L\$(NXT_ROOT)/build" \\
+		CGO_LDFLAGS="-L\$(NXT_ROOT)/build ${NXT_LIBRT}" \\
 		GOPATH=$NXT_GO_PATH \\
 		go install -v nginext
 

auto/save

@@ -16,6 +16,8 @@ NXT_MODULE_LINK='$NXT_MODULE_LINK'
 NXT_TEST_CFLAGS='$NXT_TEST_CFLAGS'
 NXT_TEST_LIBS='$NXT_TEST_LIBS'
 
+NXT_LIBRT='$NXT_LIBRT'
+
 echo=$NXT_BUILD_DIR/echo
 
 NXT_LIB_AUX_CFLAGS=

auto/shmem

@@ -13,6 +13,7 @@ nxt_feature_libs=
 
 if [ "$NXT_SYSTEM" = "Linux" ]; then
     nxt_feature_libs=-lrt
+    NXT_LIBRT=-lrt
 fi
 
 nxt_feature_test="#include <sys/mman.h>