fix #H4287 - inconsistent handling of CPPFLAGS

[Subject should mention Unix, but would exceed 50 characters.]

Explicit build rules ignore $(CPPFLAGS), but the implicit C rule
(at least in GNU make) specifies it.  If user has a value for this
in the environment, that value would apply to building some source
files but not others.  This patch gives it an explicit empty value,
so building via implicit rule should expand it to nothing and match
the fact that it's omitted from explicit rules.

There was one C++ file which relied on the implicit C++ rule.  I've
added it to the files processed by 'make depend' and re-run that.
It now will get built via an explicit rule.

Also, a small amount of reformatting for HACKCSRC.

doc/fixes36.1

@@ -221,6 +221,9 @@ unix: options file with CR+LF line ends and an invalid option line resulted in
 unix: race condition with parallel make: src/Makefile might start building
 	makedefs before finishing monst.o and objects.o; if so, util/Makefile
 	can build them too, and they might be clobbered by simultaneous write
+unix: Makefile.{src,utl} ignored CPPFLAGS which is expanded by the default .c
+	rule by GNU make so if user had a value for that in the environment,
+	it would apply to some files but not others; explicitly override it
 win32gui: getversionstring() was overflowing the provided Help About buffer
 win32gui: guard against buffer overflow in in mswin_getlin()
 win32gui: handle menu_color attribute