Improve makefiles

Added the use of the $(MAKE) variable to support 'gnumake'.
Fixed an issue with some make compile rule and the -o option.
All targets are removed from the build directory when running 'make clean'.

gtest/Makefile

@@ -10,7 +10,7 @@ SOURCES=gtest-all.cc gtest-main.cc
 $(BUILD_DIR)/%.o: %.cc
 	@echo 'Building file: $<'
 	@echo 'Invoking: GCC C++ Compiler'
-	g++ -I../ -O0 -g3 -Wall -c -o"$@" "$<"
+	g++ -I../ -O0 -g3 -Wall -c -o "$@" "$<"
 	@echo 'Finished building: $<'
 	@echo ' '