This is a quick documentation change/clarification, as this
confused me before: in my own cgo-based projects, I currently have
identical #cgo directives in each relevant source file, and I notice
with go build -x that cgo is combining the directives, leading to
pkg-config invocations with the same package name (gtk+-3.0, in my
case) repeated several times, or on Mac OS X, LDFLAGS listing
-framework Foundation -framework AppKit multiple times. Since I am
about to add a CFLAGS as well, I checked the source to cmd/cgo and
go/build (where the work is actually done) to see if that still holds
true there. Hopefully other people who have made the same mistake I
have (I don't know if anyone has) can remove the excess declarations
now; this should make things slightly easier to manage as well.

LGTM=iant
R=golang-codereviews, gobot, iant
CC=golang-codereviews
https://golang.org/cl/91520046