fine tune cmake find_package usage

cmake/benchmark.cmake

@@ -26,11 +26,12 @@ if("${gRPC_BENCHMARK_PROVIDER}" STREQUAL "module")
       message(WARNING "gRPC_BENCHMARK_PROVIDER is \"module\" but BENCHMARK_ROOT_DIR is wrong")
   endif()
 elseif("${gRPC_BENCHMARK_PROVIDER}" STREQUAL "package")
-  find_package(benchmark REQUIRED)
+  # Use "CONFIG" as there is no built-in cmake module for benchmark.
+  find_package(benchmark REQUIRED CONFIG)
   if(TARGET benchmark::benchmark)
     set(_gRPC_BENCHMARK_LIBRARIES benchmark::benchmark)
     # extract the include dir from target's properties
     get_target_property(_gRPC_BENCHMARK_INCLUDE_DIR benchmark::benchmark INTERFACE_INCLUDE_DIRECTORIES)
   endif()
-  set(_gRPC_FIND_BENCHMARK "if(NOT benchmark_FOUND)\n  find_package(benchmark)\nendif()")
+  set(_gRPC_FIND_BENCHMARK "if(NOT benchmark_FOUND)\n  find_package(benchmark CONFIG)\nendif()")
 endif()

cmake/cares.cmake

@@ -30,6 +30,7 @@ if("${gRPC_CARES_PROVIDER}" STREQUAL "module")
     set(gRPC_INSTALL FALSE)
   endif()
 elseif("${gRPC_CARES_PROVIDER}" STREQUAL "package")
+  # Use "CONFIG" as there is no built-in cmake module for c-ares.
   find_package(c-ares REQUIRED CONFIG)
   if(TARGET c-ares::cares)
     set(_gRPC_CARES_LIBRARIES c-ares::cares)

cmake/gflags.cmake

@@ -26,10 +26,11 @@ if("${gRPC_GFLAGS_PROVIDER}" STREQUAL "module")
     message(WARNING "gRPC_GFLAGS_PROVIDER is \"module\" but GFLAGS_ROOT_DIR is wrong")
   endif()
 elseif("${gRPC_GFLAGS_PROVIDER}" STREQUAL "package")
-  find_package(gflags REQUIRED)
+  # Use "CONFIG" as there is no built-in cmake module for gflags.
+  find_package(gflags REQUIRED CONFIG)
   if(TARGET gflags::gflags)
     set(_gRPC_GFLAGS_LIBRARIES gflags::gflags)
     set(_gRPC_GFLAGS_INCLUDE_DIR ${GFLAGS_INCLUDE_DIR})
   endif()
-  set(_gRPC_FIND_GFLAGS "if(NOT gflags_FOUND)\n  find_package(gflags)\nendif()")
+  set(_gRPC_FIND_GFLAGS "if(NOT gflags_FOUND)\n  find_package(gflags CONFIG)\nendif()")
 endif()

cmake/ssl.cmake

@@ -31,6 +31,10 @@ if("${gRPC_SSL_PROVIDER}" STREQUAL "module")
     set(gRPC_INSTALL FALSE)
   endif()
 elseif("${gRPC_SSL_PROVIDER}" STREQUAL "package")
+  # OpenSSL installation directory can be configured by setting OPENSSL_ROOT_DIR
+  # We expect to locate OpenSSL using the built-in cmake module as the openssl
+  # project itself does not provide installation support in its CMakeLists.txt
+  # See https://cmake.org/cmake/help/v3.6/module/FindOpenSSL.html
   find_package(OpenSSL REQUIRED)
   
   if(TARGET OpenSSL::SSL)

cmake/zlib.cmake

@@ -33,6 +33,12 @@ if("${gRPC_ZLIB_PROVIDER}" STREQUAL "module")
     set(gRPC_INSTALL FALSE)
   endif()
 elseif("${gRPC_ZLIB_PROVIDER}" STREQUAL "package")
+  # zlib installation directory can be configured by setting ZLIB_ROOT
+  # We allow locating zlib using both "CONFIG" and "MODULE" as the expectation
+  # is that many Linux systems will have zlib installed via a distribution
+  # package ("MODULE"), while on Windows the user is likely to have installed
+  # zlib using cmake ("CONFIG").
+  # See https://cmake.org/cmake/help/v3.6/module/FindZLIB.html
   find_package(ZLIB REQUIRED)
 
   if(TARGET ZLIB::ZLIB)