PR #1699: Add option to build with MSVC static runtime

Imported from GitHub PR #1699

The correct CMake policy is set, but there is no option to switch the msvc runtime when configuring the project.

Since MSVC is a multiconfiguration build tool, we need to set this CMake variable internal to the project to ensure that all of the supported build configurations get properly set (i.e. `Release` builds use `/MT` or `/MD` and `Debug` builds use `/MTd` or `/MDd`).
I tried to explictly set these flags (i.e. -DCMAKE_CXX_FLAGS_XXX) when configuring the project, but these flags were being overwritten due to [explicitly setting compiler flags](https://github.com/abseil/abseil-cpp/blob/master/CMake/AbseilHelpers.cmake#L285).

Merge 4fbe3ae into b86d574

Merging this change closes #1699

COPYBARA_INTEGRATE_REVIEW=#1699 from aaron-bray:msvc_runtime_library 4fbe3ae
PiperOrigin-RevId: 652581337
Change-Id: I64b24127cda68b681a1cd327052150508df2c4a0

Author: aaron-bray

CMakeLists.txt

@@ -61,6 +61,15 @@ list(APPEND CMAKE_MODULE_PATH
   ${CMAKE_CURRENT_LIST_DIR}/absl/copts
 )
 
+option(ABSL_MSVC_STATIC_RUNTIME
+  "Link static runtime libraries"
+  OFF)
+if(ABSL_MSVC_STATIC_RUNTIME)
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>")
+else()
+  set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>DLL")
+endif()
+
 include(CMakePackageConfigHelpers)
 include(GNUInstallDirs)
 include(AbseilDll)