mingw-build: Build winpthreads library

Build MinGW-w64 winpthreads library, which enables pthread support on
MinGW-w64 toolchain.

pthread support is required when compiling libgcc with posix thread
model.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

mingw-build.sh

@@ -87,6 +87,16 @@ build_mingw_toolchain() {
   make install -j$(nproc)
   popd
 
+  # Build MinGW winpthreads
+  mkdir mingw-winpthreads
+  pushd mingw-winpthreads
+  ../../src/mingw-w64-v${MINGW_VERSION}/mingw-w64-libraries/winpthreads/configure \
+    --prefix=${prefix}/x86_64-w64-mingw32 \
+    --host=x86_64-w64-mingw32
+  make -j$(nproc)
+  make install
+  popd
+
   # Build final GCC
   pushd gcc
   make -j$(nproc)