Adds -pthread and changes -I to -isystem in gtest's build instructions.

make/Makefile

@@ -20,10 +20,12 @@ GTEST_DIR = ..
 USER_DIR = ../samples
 
 # Flags passed to the preprocessor.
-CPPFLAGS += -I$(GTEST_DIR)/include
+# Set Google Test's header directory as a system directory, such that
+# the compiler doesn't generate warnings in Google Test headers.
+CPPFLAGS += -isystem $(GTEST_DIR)/include
 
 # Flags passed to the C++ compiler.
-CXXFLAGS += -g -Wall -Wextra
+CXXFLAGS += -g -Wall -Wextra -pthread
 
 # All tests produced by this Makefile.  Remember to add new tests you
 # created to the list.