Infer qemu path, just like in JOS

Makefile

@@ -49,6 +49,23 @@ TOOLPREFIX := $(shell if i386-jos-elf-objdump -i 2>&1 | grep '^elf32-i386$$' >/d
 	echo "***" 1>&2; exit 1; fi)
 endif
 
+# If the makefile can't find QEMU, specify its path here
+#QEMU = 
+
+# Try to infer the correct QEMU
+ifndef QEMU
+QEMU = $(shell if which qemu > /dev/null; \
+	then echo qemu; exit; \
+	else \
+	qemu=/Applications/Q.app/Contents/MacOS/i386-softmmu.app/Contents/MacOS/i386-softmmu; \
+	if test -x $$qemu; then echo $$qemu; exit; fi; fi; \
+	echo "***" 1>&2; \
+	echo "*** Error: Couldn't find a working QEMU executable." 1>&2; \
+	echo "*** Is the directory containing the qemu binary in your PATH" 1>&2; \
+	echo "*** or have you tried setting the QEMU variable in Makefile?" 1>&2; \
+	echo "***" 1>&2; exit 1)
+endif
+
 CC = $(TOOLPREFIX)gcc
 AS = $(TOOLPREFIX)gas
 LD = $(TOOLPREFIX)ld
@@ -159,27 +176,27 @@ bochs : fs.img xv6.img
 # try to generate a unique GDB port
 GDBPORT = $(shell expr `id -u` % 5000 + 25000)
 # QEMU's gdb stub command line changed in 0.11
-QEMUGDB = $(shell if qemu -help | grep -q '^-gdb'; \
+QEMUGDB = $(shell if $(QEMU) -help | grep -q '^-gdb'; \
 	then echo "-gdb tcp::$(GDBPORT)"; \
 	else echo "-s -p $(GDBPORT)"; fi)
 QEMUOPTS = -smp 2 -hdb fs.img xv6.img
 
 qemu: fs.img xv6.img
-	qemu -serial mon:stdio $(QEMUOPTS)
+	$(QEMU) -serial mon:stdio $(QEMUOPTS)
 
 qemu-nox: fs.img xv6.img
-	qemu -nographic $(QEMUOPTS)
+	$(QEMU) -nographic $(QEMUOPTS)
 
 .gdbinit: .gdbinit.tmpl
 	sed "s/localhost:1234/localhost:$(GDBPORT)/" < $^ > $@
 
 qemu-gdb: fs.img xv6.img .gdbinit
 	@echo "*** Now run 'gdb'." 1>&2
-	qemu -serial mon:stdio $(QEMUOPTS) -S $(QEMUGDB)
+	$(QEMU) -serial mon:stdio $(QEMUOPTS) -S $(QEMUGDB)
 
 qemu-nox-gdb: fs.img xv6.img .gdbinit
 	@echo "*** Now run 'gdb'." 1>&2
-	qemu -nographic $(QEMUOPTS) -S $(QEMUGDB)
+	$(QEMU) -nographic $(QEMUOPTS) -S $(QEMUGDB)
 
 # CUT HERE
 # prepare dist for students