Support pkg-config installs of libjpeg please?

[whorfin]

You may wish to solve this a different way so i didn't do a PR, but simply adding this line right after the #cgo LDFLAGS in jpeg/jpeg.go allows modern systems with pkg-config setups to work; then setting PKG_CONFIG_PATH is enough to get even weird install locations to work

#cgo LDFLAGS: -ljpeg
#cgo pkg-config: libjpeg

[edaniels]

Given that this has not been responded to, we have forked this library over to https://github.com/viam-labs/go-libjpeg/tree/fork. It includes your request @whorfin