fix: support for static builds with MSVC (googleapis#5856)

My previous cleanup was incorrect. CMake protects usage of
`CMAKE_MSVC_RUNTIME_LIBRARY` with a policy, the variable (or rather the
`MSVC_RUNTIME_LIBRARY` property set by this variable) has no effect if the
`CMP0091` policy is set to `OLD`, as would be in our builds because we say
that version 3.5 is the behavior we want in `cmake_minimum_required()`.

CMakeLists.txt

@@ -18,6 +18,11 @@ cmake_minimum_required(VERSION 3.5)
 
 # Define the project name and where to report bugs.
 set(PACKAGE_BUGREPORT "https://github.com/googleapis/google-cloud-cpp/issues")
+
+# Enable support for SelectMSVCRuntime
+if (NOT (CMAKE_VERSION VERSION_LESS 3.15))
+    cmake_policy(SET CMP0091 NEW)
+endif ()
 project(
     google-cloud-cpp
     VERSION 1.25.0
@@ -40,6 +45,9 @@ if (NOT ("${GOOGLE_CLOUD_CPP_CXX_STANDARD}" STREQUAL ""))
             "GOOGLE_CLOUD_CPP_CXX_STANDARD is retired, use CMAKE_CXX_STANDARD")
 endif ()
 
+list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
+include(SelectMSVCRuntime)
+
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
 
     # gcc-4.8 is the first release that claims C++11 support.
@@ -125,7 +133,6 @@ option(GOOGLE_CLOUD_CPP_USE_MASTER_FOR_REFDOC_LINKS
 mark_as_advanced(GOOGLE_CLOUD_CPP_USE_MASTER_FOR_REFDOC_LINKS)
 
 set(PROJECT_THIRD_PARTY_DIR "${PROJECT_SOURCE_DIR}/third_party")
-list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
 
 include(CMakeDependentOption)
 

cmake/SelectMSVCRuntime.cmake

@@ -31,8 +31,7 @@
 if (MSVC)
     # TODO(#5852) - if we require CMake >= 3.15 we can remove this conditional
     # and the hacks below.
-    if ((${CMAKE_VERSION} VERSION_GREATER 3.15) OR (${CMAKE_VERSION}
-                                                    VERSION_EQUAL 3.14))
+    if (NOT (CMAKE_VERSION VERSION_LESS 3.15))
         if (VCPKG_TARGET_TRIPLET MATCHES "-static$")
             set(CMAKE_MSVC_RUNTIME_LIBRARY
                 "MultiThreaded$<$<CONFIG:Debug>:Debug>")