Support MSYS2 MinGW64 builds (rogchap#46)

* Support MSYS2 MinGW64 builds

* Update README.md to include instructions for setting up
Windows dev environments

* Update README.md

Co-authored-by: Roger Chapman <rogchap@gmail.com>

* Switch up #cgo flags to match master

* Use correct syntax for #cgo constraints

Co-authored-by: neptoess <neptoess@gmail.com>
Co-authored-by: Roger Chapman <rogchap@gmail.com>

README.md

@@ -91,6 +91,18 @@ are included for Linux and OSX ie. you *should not* require to build V8 yourself
 
 V8 requires 64-bit, therefore will not work on 32-bit systems. 
 
+## Windows
+While no prebuilt static V8 library is included for Windows, MSYS2 provides a package containing
+a dynamically linked V8 library that works.
+
+To set this up:
+1. Install MSYS2 (https://www.msys2.org/)
+2. Add the Mingw-w64 bin to your PATH environment variable (`C:\msys64\mingw64\bin` by default)
+3. Open MSYS2 MSYS and execute `pacman -S mingw-w64-x86_64-toolchain mingw-w64-x86_64-v8`
+4. This will allow building projects that depend on `v8go`, but, in order to actually run them,
+   you will need to copy the `snapshot_blob.bin` file from the Mingw-w64 bin folder to your program's
+   working directory (which is typically wherever `main.go` is)
+
 ---
 
 V8 Gopher image based on original artwork from the amazing [Renee French](http://reneefrench.blogspot.com).

cgo.go

@@ -1,7 +1,9 @@
 package v8go
 
-// #cgo CXXFLAGS: -fno-rtti -fpic -std=c++14 -I${SRCDIR}/deps/include -DV8_COMPRESS_POINTERS -DV8_31BIT_SMIS_ON_64BIT_ARCH
+// #cgo CXXFLAGS: -fno-rtti -fpic -std=c++14 -DV8_COMPRESS_POINTERS -DV8_31BIT_SMIS_ON_64BIT_ARCH
+// #cgo darwin linux CXXFLAGS: -I${SRCDIR}/deps/include
 // #cgo LDFLAGS: -pthread -lv8
+// #cgo windows LDFLAGS: -lv8_libplatform
 // #cgo darwin LDFLAGS: -L${SRCDIR}/deps/darwin-x86_64
 // #cgo linux LDFLAGS: -L${SRCDIR}/deps/linux-x86_64
 import "C"

v8go.cc

@@ -85,6 +85,9 @@ extern "C"
 /********** Isolate **********/
 
 void Init() {
+#ifdef _WIN32
+    V8::InitializeExternalStartupData(".");
+#endif
     V8::InitializePlatform(default_platform.get());
     V8::Initialize();
     return;