Adding a helpful installation message about protobuf.

Also, fixing protobuf in-place compilation for real, this time.
diff --git a/Makefile b/Makefile
index 1fc78b6..ae5d5d4 100644
--- a/Makefile
+++ b/Makefile
@@ -234,6 +234,7 @@
 ZLIB_CHECK_CMD = $(CC) $(CFLAGS) $(CPPFLAGS) -o /dev/null test/build/zlib.c -lz $(LDFLAGS)
 PERFTOOLS_CHECK_CMD = $(CC) $(CFLAGS) $(CPPFLAGS) -o /dev/null test/build/perftools.c -lprofiler $(LDFLAGS)
 PROTOBUF_CHECK_CMD = $(CXX) $(CXXFLAGS) $(CPPFLAGS) -o /dev/null test/build/protobuf.cc -lprotobuf $(LDFLAGS)
+PROTOC_CMD = which protoc
 PROTOC_CHECK_CMD = protoc --version | grep -q libprotoc.3
 
 ifndef REQUIRE_CUSTOM_LIBRARIES_$(CONFIG)
@@ -244,10 +245,11 @@
 endif
 endif
 
+HAS_SYSTEM_PROTOBUF_VERIFY = $(shell $(PROTOBUF_CHECK_CMD) 2> /dev/null && echo true || echo false)
 ifndef REQUIRE_CUSTOM_LIBRARIES_$(CONFIG)
 HAS_SYSTEM_OPENSSL_ALPN = $(shell $(OPENSSL_ALPN_CHECK_CMD) 2> /dev/null && echo true || echo false)
 HAS_SYSTEM_ZLIB = $(shell $(ZLIB_CHECK_CMD) 2> /dev/null && echo true || echo false)
-HAS_SYSTEM_PROTOBUF = $(shell $(PROTOBUF_CHECK_CMD) 2> /dev/null && echo true || echo false)
+HAS_SYSTEM_PROTOBUF = $(HAS_SYSTEM_PROTOBUF_VERIFY)
 else
 # override system libraries if the config requires a custom compiled library
 HAS_SYSTEM_OPENSSL_ALPN = false
@@ -255,7 +257,12 @@
 HAS_SYSTEM_PROTOBUF = false
 endif
 
+HAS_PROTOC = $(shell $(PROTOC_CMD) && echo true || echo false)
+ifeq ($(HAS_PROTOC),true)
 HAS_VALID_PROTOC = $(shell $(PROTOC_CHECK_CMD) 2> /dev/null && echo true || echo false)
+else
+HAS_VALID_PROTOC = false
+endif
 
 ifeq ($(wildcard third_party/openssl/ssl/ssl.h),)
 HAS_EMBEDDED_OPENSSL_ALPN = false
@@ -327,6 +334,13 @@
 NO_DEPS = true
 endif
 
+INSTALL_OK = false
+ifeq ($(HAS_VALID_PROTOC),true)
+ifeq ($(HAS_SYSTEM_PROTOBUF_VERIFY),true)
+INSTALL_OK = true
+endif
+endif
+
 .SECONDARY = %.pb.h %.pb.cc
 
 PROTOC_PLUGINS = $(BINDIR)/$(CONFIG)/grpc_cpp_plugin $(BINDIR)/$(CONFIG)/grpc_python_plugin $(BINDIR)/$(CONFIG)/grpc_ruby_plugin
@@ -880,7 +894,7 @@
 
 $(LIBDIR)/$(CONFIG)/protobuf/libprotobuf.a: third_party/protobuf/configure
 	$(E) "[MAKE]    Building protobuf"
-	$(Q)(cd third_party/protobuf ; CC="$(CC)" CPPFLAGS="-fPIC" CXX="$(CXX)" LDFLAGS="$(LDFLAGS_$(CONFIG)) -g" CXXFLAGS="-DLANG_CXX11 -std=c++11" CPPFLAGS="$(CPPFLAGS_$(CONFIG)) -g" ./configure --disable-shared --enable-static)
+	$(Q)(cd third_party/protobuf ; CC="$(CC)" CXX="$(CXX)" LDFLAGS="$(LDFLAGS_$(CONFIG)) -g" CXXFLAGS="-DLANG_CXX11 -std=c++11" CPPFLAGS="-fPIC $(CPPFLAGS_$(CONFIG)) -g" ./configure --disable-shared --enable-static)
 	$(Q)$(MAKE) -C third_party/protobuf clean
 	$(Q)$(MAKE) -C third_party/protobuf
 	$(Q)mkdir -p $(LIBDIR)/$(CONFIG)/protobuf
@@ -1915,7 +1929,7 @@
 	$(Q) $(CXX) $(CXXFLAGS) $(CPPFLAGS) -MMD -MF $(addsuffix .dep, $(basename $@)) -c -o $@ $<
 
 
-install: install_c install_cxx install-plugins
+install: install_c install_cxx install-plugins verify-install
 
 install_c: install-headers_c install-static_c install-shared_c
 
@@ -2059,6 +2073,25 @@
 	$(Q) $(INSTALL) $(BINDIR)/$(CONFIG)/grpc_ruby_plugin $(prefix)/bin/grpc_ruby_plugin
 endif
 
+verify-install:
+ifeq ($(SYSTEM_OK),true)
+	@echo "Your system looks ready to go."
+	@echo
+else
+	@echo "Your system doesn't have protoc 3.0.0+ installed. While this"
+	@echo "won't prevent grpc from working, you won't be able to compile"
+	@echo "and run any meaningful code with it."
+	@echo
+	@echo
+	@echo "Please download and install protobuf 3.0.0+ from:"
+	@echo
+	@echo "   https://github.com/google/protobuf/releases"
+	@echo
+	@echo "Once you've done so, you can re-run this check by doing:"
+	@echo
+	@echo "   make verify-install"
+endif
+
 clean:
 	$(E) "[CLEAN]   Cleaning build directories."
 	$(Q) $(RM) -rf $(OBJDIR) $(LIBDIR) $(BINDIR) $(GENDIR)