Add build tests to verify correct compilation of as many modules as possible under as many architectures as possible.

Fix broken compilation of LUFA under C++ compilers when the Serial peripheral module header file is included in a C++ source file.

Fix missing semicolon in the UC3 architecture host pipe functions.

Fix failed compilation for the XMEGA architecture if USB_DEVICE_ONLY us not specified.

makefile

@@ -21,6 +21,7 @@ all:
 	$(MAKE) -C Demos $@ -s
 	$(MAKE) -C Projects $@ -s
 	$(MAKE) -C Bootloaders $@ -s
+	$(MASK) -C BuildTests $@ -s
 	@echo
 	@echo LUFA \"make $@\" operation complete.