doc: clarify that any C compiler works for cgo

doc: clarify that any C compiler works for cgo

Currently the footnote says "gcc is required only if you plan to use cgo",
but the footnote was referenced from the text:

   "use the clang or gcc† that comes with Xcode‡ for cgo support"

That seems to imply that clang doesn't get you cgo support on OS X,
which isn't true. The update text matches what the install-source.html
page says.

Change-Id: Ib88464a0d138227d357033123f6675a77d5d777f
Reviewed-on: https://go-review.googlesource.com/28786
Reviewed-by: Minux Ma <minux@golang.org>