[libcxx] Respect CMAKE_MSVC_RUNTIME_LIBRARY wrt whether to use the debug CRT

If CMAKE_MSVC_RUNTIME_LIBRARY isn't set, then CMake defaults to
the debug CRT, if CMAKE_BUILD_TYPE is set to Debug. If
CMAKE_MSVC_RUNTIME_LIBRARY is set though, that overrides any
implicit defaults.

Match this in libcxx's own manual linking logic. This allows
decoupling the debug CRT from the CMAKE_BUILD_TYPE and allows users
to configure their builds exactly how they want.

Differential Revision: https://reviews.llvm.org/D155561

Author: mstorsjo

libcxx/CMakeLists.txt

@@ -682,7 +682,8 @@ function(cxx_link_system_libraries target)
   endif()
 
   if (MSVC)
-    if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
+    if ((NOT CMAKE_MSVC_RUNTIME_LIBRARY AND uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
+        OR (CMAKE_MSVC_RUNTIME_LIBRARY MATCHES "Debug"))
       set(LIB_SUFFIX "d")
     else()
       set(LIB_SUFFIX "")

libcxx/test/CMakeLists.txt

@@ -59,7 +59,8 @@ if (MSVC)
     set(cxx_lib "libcpmt")
   endif()
 
-  if (uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
+  if ((NOT CMAKE_MSVC_RUNTIME_LIBRARY AND uppercase_CMAKE_BUILD_TYPE STREQUAL "DEBUG")
+      OR (CMAKE_MSVC_RUNTIME_LIBRARY MATCHES "Debug"))
     set(dbg_include " -D_DEBUG -include set_windows_crt_report_mode.h")
     set(crt_lib "${crt_lib}d")
     set(cxx_lib "${cxx_lib}d")