grpc-1.0.1 Makefile simple issue breaks MSYS2 execution

[emmenlau]

The Makefile of grpc-1.0.1 executes protoc with argument:

--plugin=protoc-gen-grpc=$(BINDIR)/$(CONFIG)/grpc_cpp_plugin

However when used on Windows with MSYS2, the correct name of grpc_cpp_plugin is grpc_cpp_plugin.exe. Without the exe-suffix, the plugin fails. With the exe-suffix, everything works as expected. Would it be possible to add a target-specific suffix for Windows?

[jtattermusch]

Fixed in #9307.