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

README

@@ -119,21 +119,22 @@ and Xcode) to compile
 
   ${GTEST_DIR}/src/gtest-all.cc
 
-with
-
-  ${GTEST_DIR}/include and ${GTEST_DIR}
-
-in the header search path.  Assuming a Linux-like system and gcc,
+with ${GTEST_DIR}/include in the system header search path and ${GTEST_DIR}
+in the normal header search path.  Assuming a Linux-like system and gcc,
 something like the following will do:
 
-  g++ -I${GTEST_DIR}/include -I${GTEST_DIR} -c ${GTEST_DIR}/src/gtest-all.cc
+  g++ -isystem ${GTEST_DIR}/include -I${GTEST_DIR} \
+      -pthread -c ${GTEST_DIR}/src/gtest-all.cc
   ar -rv libgtest.a gtest-all.o
 
-Next, you should compile your test source file with
-${GTEST_DIR}/include in the header search path, and link it with gtest
-and any other necessary libraries:
+(We need -pthread as Google Test uses threads.)
 
-  g++ -I${GTEST_DIR}/include path/to/your_test.cc libgtest.a -o your_test
+Next, you should compile your test source file with
+${GTEST_DIR}/include in the system header search path, and link it
+with gtest and any other necessary libraries:
+
+  g++ -isystem ${GTEST_DIR}/include -pthread path/to/your_test.cc libgtest.a \
+      -o your_test
 
 As an example, the make/ directory contains a Makefile that you can
 use to build Google Test on systems where GNU make is available