Allow overriding default name of `pkg-config` tool via environment
variable PKG_CONFIG (same as used by autoconf pkg.m4 macros). This
facilitates easy cross-compilation of cgo code.

Original patch against Go <= 1.4 was written by
xnox_canonical <dimitri.ledkov@canonical.com> in 2014.
Source: https://codereview.appspot.com/104960043/

Fixes #16253

Change-Id: I31c33ffc3ecbff65da31421e6188d092ab4fe7e4
Reviewed-on: https://go-review.googlesource.com/29991Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>