go get failed

go get github.com/xlab/opus-go/opus
# pkg-config --cflags opus opus opus opus
Package opus was not found in the pkg-config search path.
Perhaps you should add the directory containing `opus.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opus' found
Package opus was not found in the pkg-config search path.
Perhaps you should add the directory containing `opus.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opus' found
Package opus was not found in the pkg-config search path.
Perhaps you should add the directory containing `opus.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opus' found
Package opus was not found in the pkg-config search path.
Perhaps you should add the directory containing `opus.pc'
to the PKG_CONFIG_PATH environment variable
No package 'opus' found
pkg-config: exit status 1