Makefile: Use implicit rules for compiling C, honour flags

[nicoonoclaste]

In addition to being shorter, this does the right thing with regards to
honouring various flags: not only CFLAGS and LDFLAGS, but also CPPFLAGS.

CPPFLAGS are the flags passed to cpp (the C PreProcessor); for example,
we use -D_FORTIFY_SOURCE=2 in Debian for build hardening.

[skeeto]

Thanks! In all these years I had not noticed that Debian has a separate CPPFLAGS argument. However, POSIX make has no default inference rule in this case. It's a common extension, but, even when present, the various make implementations define it differently. FreeBSD make doesn't include CPPFLAGS, and the macro order varies among others. Even more, none of them explicitly document this default inference rule, not even GNU make (which only says "does the right thing"). Instead I explicitly added CPPFLAGS to the rule so that it's portable and consistent.