go version go1.10.3 darwin/amd64

GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/jpap/Library/Caches/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/jpap/go"
GORACE=""
GOROOT="/usr/local/Cellar/go/1.10.3/libexec"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.10.3/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/f8/2x23x3n92nb_mjyj10v_ssph0000gn/T/go-build284257526=/tmp/go-build -gno-record-gcc-switches -fno-common"

lib/lib.go
lib+main.go

// lib/lib.go
package lib

import (
	"C"
	"fmt"
)

//export ExportedFromLib
func ExportedFromLib() {
	fmt.Println("ExportedFromLib")
}

func init() {
	fmt.Println("lib.init")
}

// lib+main.go
package main

import (
	"C"
	"fmt"

	_ "github.com/exporttest/lib"
)

//export ExportedFromMain
func ExportedFromMain() {
	fmt.Println("ExportedFromMain")
}

func main() {
	// Empty, but required for buildmode=c-shared
}

// test.c
#include "libtest.h"
#include <stdio.h>

// Declared here as it does not exist in generated libtest.h
extern void ExportedFromLib();

int main() {
	ExportedFromLib();
	return 0;
}

$ gcc -o test test.c -L. -ltest
$ ./test
lib.init
ExportedFromLib
$