Add cross build script

.travis/cross_build.sh

@@ -0,0 +1,68 @@
+#!/usr/bin/env bash
+
+# Functions.
+function info() {
+    echo -e "\033[00;34mi\033[0m $1"
+}
+
+function fail() {
+    echo -e "\033[00;31m!\033[0m $1"
+    exit 1
+}
+
+function build() {
+    goos=$1
+    goarch=$2
+
+    info "Building for $goos $goarch..."
+    GOOS=$goos GOARCH=$goarch go build -o $goos_$goarch main.go
+    if [[ $? -ne 0 ]]; then
+        fail "Could not build for $goos $goarch. Aborting."
+    fi
+}
+
+# Create build directory.
+mkdir -p bin
+cd bin
+
+# Create go.mod
+cat <<EOF > go.mod
+module cross_build
+require github.com/unidoc/unipdf/v3 v3.0.0
+EOF
+
+echo "replace github.com/unidoc/unipdf/v3 => $TRAVIS_BUILD_DIR" >> go.mod
+
+# Create Go file.
+cat <<EOF > main.go
+package main
+
+import (
+	_ "github.com/unidoc/unipdf/v3/annotator"
+	_ "github.com/unidoc/unipdf/v3/common"
+	_ "github.com/unidoc/unipdf/v3/common/license"
+	_ "github.com/unidoc/unipdf/v3/contentstream"
+	_ "github.com/unidoc/unipdf/v3/contentstream/draw"
+	_ "github.com/unidoc/unipdf/v3/core"
+	_ "github.com/unidoc/unipdf/v3/core/security"
+	_ "github.com/unidoc/unipdf/v3/core/security/crypt"
+	_ "github.com/unidoc/unipdf/v3/creator"
+	_ "github.com/unidoc/unipdf/v3/extractor"
+	_ "github.com/unidoc/unipdf/v3/fdf"
+	_ "github.com/unidoc/unipdf/v3/fjson"
+	_ "github.com/unidoc/unipdf/v3/model"
+	_ "github.com/unidoc/unipdf/v3/model/optimize"
+	_ "github.com/unidoc/unipdf/v3/model/sighandler"
+	_ "github.com/unidoc/unipdf/v3/ps"
+	_ "github.com/unidoc/unipdf/v3/render"
+)
+
+func main() {}
+EOF
+
+# Build file.
+for os in "linux" "darwin" "windows"; do
+    for arch in "amd64"; do
+        build $os $arch
+    done
+done