(see https://golang.org/pkg/go/build/#hdr-Build_Constraints for details about the constraint syntax).
For example:

	// #cgo CFLAGS: -DPNG_DEBUG=1
	// #cgo amd64 386 CFLAGS: -DX86=1
	// #cgo LDFLAGS: -lpng
	// #include <png.h>
	import "C"

Alternatively, CPPFLAGS and LDFLAGS may be obtained via the pkg-config tool
using a '#cgo pkg-config:' directive followed by the package names.
For example:

lib.ok.us
lib.or.us
lib.pa.us
lib.pr.us
lib.ri.us
lib.sc.us
lib.sd.us
lib.tn.us
lib.tx.us
lib.ut.us
lib.va.us
lib.vi.us
lib.vt.us
lib.wa.us
lib.wi.us
lib.wy.us
libp2p.direct
lidl
lier.no
lierne.no
life
lifeinsurance
lifestyle
lig.it
lighting
liguria.it
like
likes-pie.com
likescandy.com
lillehammer.no
lillesand.no
lilly

  `They couldn't have done that, you know,' Alice gently
remarked; `they'd have been ill.'

  `So they were,' said the Dormouse; `VERY ill.'

  Alice tried to fancy to herself what such an extraordinary ways
of living would be like, but it puzzled her too much, so she went
on:  `But why did they live at the bottom of a well?'

  `Take some more tea,' the March Hare said to Alice, very
earnestly.

0EA0          ; valid                                  # 12.0 LAO LETTER PALI BHA
0EA1..0EA3    ; valid                                  # 1.1  LAO LETTER MO..LAO LETTER LO LING
0EA4          ; disallowed                             # NA   <reserved-0EA4>
0EA5          ; valid                                  # 1.1  LAO LETTER LO LOOT