Set gRPC_INSTALL to ON by default for MSVC and OFF if a subproject

Now that gRPC_INSTALL is forced off if using bundled third-party
libraries, it no longer makes sense to set it OFF by default for MSVC.

However, we do want to set it OFF by default if gRPC is being built as a
subproject of another project.

CMakeLists.txt

@@ -49,7 +49,8 @@ project(${PACKAGE_NAME} C CXX)
 option(gRPC_BUILD_TESTS "Build tests" OFF)
 
 set(gRPC_INSTALL_default ON)
-if (MSVC)
+if (NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+  # Disable gRPC_INSTALL by default if building as a submodule
   set(gRPC_INSTALL_default OFF)
 endif()
 set(gRPC_INSTALL ${gRPC_INSTALL_default} CACHE BOOL

templates/CMakeLists.txt.template

@@ -93,7 +93,8 @@
   option(gRPC_BUILD_TESTS "Build tests" OFF)
 
   set(gRPC_INSTALL_default ON)
-  if (MSVC)
+  if (NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+    # Disable gRPC_INSTALL by default if building as a submodule
     set(gRPC_INSTALL_default OFF)
   endif()
   set(gRPC_INSTALL <%text>${gRPC_INSTALL_default}</%text> CACHE BOOL