Add gtest_force_shared_crt for MSVC (census-instrumentation#284)

This fixes linking with MSVC.

Author: meastp

cmake/OpenCensusDeps.cmake

@@ -16,6 +16,19 @@ if(BUILD_TESTING)
   if(NOT TARGET gtest_main)
     message(STATUS "Dependency: googletest (BUILD_TESTING=${BUILD_TESTING})")
 
+    if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+      # All the libraries in the build must use either /MD or /MT (runtime
+      # library to link)
+      #
+      # force this option to ON so that Google Test will use /MD instead of /MT
+      # /MD is now the default for Visual Studio, so it should be our default,
+      # too
+      option(
+        gtest_force_shared_crt
+        "Use shared (DLL) run-time lib even when Google Test is built as static lib."
+        ON)
+    endif()
+
     configure_file(${CMAKE_CURRENT_SOURCE_DIR}/cmake/googletest.CMakeLists.txt
                    ${CMAKE_BINARY_DIR}/googletest-download/CMakeLists.txt)
     execute_process(COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}" .