contrib: protobuf: don't use pthread on Win32

It has its own internal Win32 threads support. No need to rebuild this (heavy)
contrib when pthread changes.

No need to use winpthread from mingw either in the native protoc build when it
can use the Win32 threading API. And thus the codebase is the same in both
case.

contrib/src/protobuf/protobuf-no-mingw-pthread.patch

+--- protobuf/m4/acx_pthread.m4.orig	2022-02-23 11:15:51.818483000 +0100
++++ protobuf/m4/acx_pthread.m4	2022-02-23 11:16:06.148340600 +0100
+@@ -134,6 +134,10 @@ case "${host_cpu}-${host_os}" in
+ 
+         acx_pthread_flags="-pthreads pthread -mt -pthread $acx_pthread_flags"
+         ;;
++        *mingw*)
++        # Do not use pthread, fallback to the win32 threading API
++        acx_pthread_flags=""
++        ;;
+ esac
+ 
+ if test x"$acx_pthread_ok" = xno; then

contrib/src/protobuf/rules.mak

@@ -24,9 +24,6 @@ $(TARBALLS)/protoc-$(PROTOBUF_VERSION)-cpp.tar.gz: $(TARBALLS)/protobuf-$(PROTOB
 .sum-protobuf: protobuf-$(PROTOBUF_VERSION)-cpp.tar.gz
 
 DEPS_protobuf = zlib $(DEPS_zlib)
-ifdef HAVE_WIN32
-DEPS_protobuf += pthreads $(DEPS_pthreads)
-endif
 
 PROTOBUFVARS := DIST_LANG="cpp"
 PROTOCVARS := DIST_LANG="cpp"
@@ -42,6 +39,7 @@ protoc: protoc-$(PROTOBUF_VERSION)-cpp.tar.gz .sum-protoc
 	$(APPLY) $(SRC)/protobuf/protobuf-disable-gmock.patch
 	$(APPLY) $(SRC)/protobuf/protobuf-fix-build.patch
 	$(APPLY) $(SRC)/protobuf/include-algorithm.patch
+	$(APPLY) $(SRC)/protobuf/protobuf-no-mingw-pthread.patch
 	$(MOVE)
 
 .protoc: protoc
@@ -57,6 +55,7 @@ protobuf: protobuf-$(PROTOBUF_VERSION)-cpp.tar.gz .sum-protobuf
 	$(APPLY) $(SRC)/protobuf/dont-build-protoc.patch
 	$(APPLY) $(SRC)/protobuf/protobuf-fix-build.patch
 	$(APPLY) $(SRC)/protobuf/include-algorithm.patch
+	$(APPLY) $(SRC)/protobuf/protobuf-no-mingw-pthread.patch
 	$(MOVE)
 
 .protobuf: protobuf