build: explain $PWD use

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4639064

src/Make.ccmd

@@ -36,6 +36,7 @@ y.tab.c: y.tab.h
 
 all: $(TARG)
 
+# Use $(PWD)/$*.c so that gdb shows full path in stack traces.
 %.$(HOST_O): %.c
 	$(HOST_CC) $(HOST_CFLAGS) -c "$(PWD)/$*.c"
 

src/Make.clib

@@ -6,6 +6,7 @@
 
 all: $(LIB)
 
+# Use $(PWD)/$*.c so that gdb shows full path in stack traces.
 %.$(HOST_O): %.c
 	$(HOST_CC) $(HOST_CFLAGS) -c "$(PWD)/$*.c"