Added instructions for building for Windows

Author: buffer51

README.md

@@ -24,6 +24,12 @@ This tutorial aims at building the **GNU toolchain 4.9** with **Android NDK r11c
 It has been tested on Linux x86_64, but I expect that it should work with
 small changes on other systems supported by the NDK.
 
+### Windows
+
+Although the NDK supports Windows (32-bit & 64-bit variants), the
+toochain can only be built from Linux. The process is the roughly the
+same, with a few extra steps detailed along the way.
+
 ### Steps
 
 #### Requirements
@@ -44,6 +50,9 @@ On Debian-based Linux, you can run:
 sudo apt-get install git make gcc g++ m4 texinfo bison flex wget
 ```
 
+**Note:** When building the toolchain for Windows, **mingw-w64** is
+also required.
+
 #### Android NDK
 
 Download the Android NDK r11c from Google, and extract it.
@@ -62,18 +71,18 @@ The next step is to download the GNU toolchain components from Google
 repositories. We'll clone them in a folder called `toolchain/`.
 ```
 mkdir toolchain && cd toolchain
-git clone https://android.googlesource.com/toolchain/gcc
-git clone https://android.googlesource.com/toolchain/build
-git clone https://android.googlesource.com/toolchain/gmp
-git clone https://android.googlesource.com/toolchain/gdb
-git clone https://android.googlesource.com/toolchain/mpc
-git clone https://android.googlesource.com/toolchain/mpfr
-git clone https://android.googlesource.com/toolchain/expat
-git clone https://android.googlesource.com/toolchain/ppl
-git clone https://android.googlesource.com/toolchain/cloog
-git clone https://android.googlesource.com/toolchain/isl
-git clone https://android.googlesource.com/toolchain/sed
-git clone https://android.googlesource.com/toolchain/binutils
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/gcc
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/build
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/gmp
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/gdb
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/mpc
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/mpfr
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/expat
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/ppl
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/cloog
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/isl
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/sed
+git clone -b ndk-r11c https://android.googlesource.com/toolchain/binutils
 ```
 
 #### Add support for Fortran
@@ -104,7 +113,30 @@ to
 ```
 prebuilt_ndk = 'ndk'
 ```
-so that the building script can find the `ndk/platforms` folder correctly.
+so that the building script can find the `ndk/platforms` folder
+correctly.
+
+**Note:** When building the toolchain for Windows,
+you need to change `ndk/build/tools/prebuilt-common.sh` for the MinGW
+wrapper to be found. Find the line that says:
+```
+# generate wrappers for BUILD toolchain
+```
+and replace the section that follows by:
+```
+LEGACY_TOOLCHAIN_PREFIX="/usr/bin/x86_64-linux-gnu-"
+$NDK_BUILDTOOLS_PATH/gen-toolchain-wrapper.sh --src-prefix=i386-linux-gnu- \
+        --cflags="-m32" --cxxflags="-m32" --ldflags="-m elf_i386" --asflags="--32" \
+        --dst-prefix="$LEGACY_TOOLCHAIN_PREFIX" "$CROSS_WRAP_DIR"
+$NDK_BUILDTOOLS_PATH/gen-toolchain-wrapper.sh --src-prefix=i386-pc-linux-gnu- \
+        --cflags="-m32" --cxxflags="-m32" --ldflags="-m elf_i386" --asflags="--32" \
+        --dst-prefix="$LEGACY_TOOLCHAIN_PREFIX" "$CROSS_WRAP_DIR"
+# 64-bit BUILD toolchain.  libbfd is still built in 32-bit.
+$NDK_BUILDTOOLS_PATH/gen-toolchain-wrapper.sh --src-prefix=x86_64-linux-gnu- \
+        --dst-prefix="$LEGACY_TOOLCHAIN_PREFIX" "$CROSS_WRAP_DIR"
+$NDK_BUILDTOOLS_PATH/gen-toolchain-wrapper.sh --src-prefix=x86_64-pc-linux-gnu- \
+        --dst-prefix="$LEGACY_TOOLCHAIN_PREFIX" "$CROSS_WRAP_DIR"
+```
 
 **Note:** If you want to build **standalone toolchains**
 (i.e. you're not using `ndk-build`), there is one extra step.
@@ -125,14 +157,17 @@ You can specify which toolchain to build. For instance:
 See `./build.py -h` for possible values.
 If nothing is specified, it will build all of them.
 
+**Note:** When building the toolchain for Windows, add `--host windows`
+(or `--host windows64` for 64-bit).
+
 ### Deploying
 
 If you built a standalone toolchain, just extract the archive. You'll probably
 want to add `$(TOOLCHAIN)/bin` to your path.
 
 Otherwise, to allow `ndk-build` to use your new toolchain, extract the archive
-under `ndk/toolchains/$(TOOLCHAIN)/prebuilt/$(HOST_ARCH)`. Don't forget to back
+under `ndk/toolchains/$(TOOLCHAIN)/prebuilt/$(HOST_ARCH)`. Don't forget to back up
 the toolchain that was already packaged with the NDK.
 
-For instance, on Linux 64-bit for the AArch64 toolchain, unpack the archive as
+For instance, on Linux x86_64 for the AArch64 toolchain, unpack the archive as
 `ndk/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64`.