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

@@ -82,7 +82,8 @@ build_mingw_toolchain() {
   ../../src/mingw-w64-v${MINGW_VERSION}/configure \
     --prefix=${prefix}/x86_64-w64-mingw32 \
     --host=x86_64-w64-mingw32 \
-    --with-default-msvcrt=ucrt
+    --with-default-msvcrt=ucrt \
+    --with-libraries=winpthreads
   make -j$(nproc)
   make install -j$(nproc)
   popd