Use gRPC_BUILD_MSVC_MP_COUNT for windows cmake distrib tests (#28202)

CMakeLists.txt

@@ -42,7 +42,7 @@ set(gRPC_INSTALL_LIBDIR "lib" CACHE STRING "Installation directory for libraries
 set(gRPC_INSTALL_INCLUDEDIR "include" CACHE STRING "Installation directory for headers")
 set(gRPC_INSTALL_CMAKEDIR "lib/cmake/${PACKAGE_NAME}" CACHE STRING "Installation directory for cmake config files")
 set(gRPC_INSTALL_SHAREDIR "share/grpc" CACHE STRING "Installation directory for root certificates")
-set(gRPC_BUILD_MSVC_MP_COUNT 0 CACHE INTERNAL "The maximum number of processes for MSVC /MP option")
+set(gRPC_BUILD_MSVC_MP_COUNT 0 CACHE STRING "The maximum number of processes for MSVC /MP option")
 
 # Options
 option(gRPC_BUILD_TESTS "Build tests" OFF)

examples/cpp/helloworld/cmake_externalproject/CMakeLists.txt

@@ -100,6 +100,7 @@ ExternalProject_Add(grpc
   CMAKE_CACHE_ARGS
         -DgRPC_INSTALL:BOOL=ON
         -DgRPC_BUILD_TESTS:BOOL=OFF
+        -DgRPC_BUILD_MSVC_MP_COUNT:STRING=-1
         -DgRPC_PROTOBUF_PROVIDER:STRING=package
         -DgRPC_PROTOBUF_PACKAGE_TYPE:STRING=CONFIG
         -DProtobuf_DIR:PATH=${_FINDPACKAGE_PROTOBUF_CONFIG_DIR}

templates/CMakeLists.txt.template

@@ -164,7 +164,7 @@
   set(gRPC_INSTALL_INCLUDEDIR "include" CACHE STRING "Installation directory for headers")
   set(gRPC_INSTALL_CMAKEDIR "lib/cmake/<%text>${PACKAGE_NAME}</%text>" CACHE STRING "Installation directory for cmake config files")
   set(gRPC_INSTALL_SHAREDIR "share/grpc" CACHE STRING "Installation directory for root certificates")
-  set(gRPC_BUILD_MSVC_MP_COUNT 0 CACHE INTERNAL "The maximum number of processes for MSVC /MP option")
+  set(gRPC_BUILD_MSVC_MP_COUNT 0 CACHE STRING "The maximum number of processes for MSVC /MP option")
 
   # Options
   option(gRPC_BUILD_TESTS "Build tests" OFF)

test/distrib/cpp/run_distrib_test_cmake.bat

@@ -78,6 +78,7 @@ cmake ^
   -DZLIB_ROOT=%INSTALL_DIR% ^
   -DgRPC_INSTALL=ON ^
   -DgRPC_BUILD_TESTS=OFF ^
+  -DgRPC_BUILD_MSVC_MP_COUNT=-1 ^
   -DgRPC_ABSL_PROVIDER=package ^
   -DgRPC_CARES_PROVIDER=package ^
   -DgRPC_PROTOBUF_PROVIDER=package ^