Bug #26399073: MYSQL DOES NOT COMPILE WITH CLANG ON WINDOWS [noclose]

Modify convenience library linking so that one can use lld-link for linking
executables and use MSVC's linker for linking convenience libraries. This is
currently the best of both worlds, as lld-link can't link convenience libraries,
and MSVC's linker has a hard-coded symbol/file limit that chokes on Clang's
output in debug mode.

In sum, this means that it's possible to compile and link MySQL with Clang on
Windows also in debug mode.

Change-Id: If62cadff0db8f3ce80baf18043c4732458319d24

Author: Steinar H. Gunderson

BUILD/README

@@ -39,6 +39,22 @@ these cmake variables off:
 WITH_DEFAULT_COMPILER_OPTIONS
 WITH_DEFAULT_FEATURE_SET
 
+===
+
+It is possible to compile with Clang on Windows, which compiles faster
+and also yields faster binaries, but this is experimental and not officially
+supported. To compile, use LLVM 5.0.0 or newer from releases.llvm.org
+(Clang/C2, which is Microsoft's own version of Clang, is not supported),
+and put a Windows compile of Ninja in your path. Then execute the following
+in a shell:
+
+  "c:\Program Files (x86)\Microsoft Visual Studio\2017\Enterprise\VC\Auxiliary\Build\vcvarsall.bat" amd64
+  set PATH=<path to Ninja>;c:\program files\llvm\msbuild-bin;%PATH%
+  "c:\Program Files\CMake 3.8\bin\cmake.exe" -G Ninja -DCMAKE_C_COMPILER="c:/program files/llvm/msbuild-bin/cl.exe" -DCMAKE_CXX_COMPILER="c:/program files/llvm/msbuild-bin/cl.exe" -DCMAKE_LINKER="c:/program files/llvm/bin/lld-link.exe" -DWITH_BOOST=<path to Boost> ..
+  ninja
+
+===
+
 Note: For building with unit tests, you need to tell cmake where to find
 the sources, see ../unittest/gunit/CMakeLists.txt
 You should *not* 'make install' googletest/googlemock on your system,

cmake/merge_archives.cmake.in

@@ -76,10 +76,19 @@ EXECUTE_PROCESS(
 
 IF(MSVC)
   # See CMAKE_CXX_CREATE_STATIC_LIBRARY
-  EXECUTE_PROCESS(
-    COMMAND ${CMAKE_LINKER} "/lib" "/NOLOGO" "${CMAKE_STATIC_LINKER_FLAGS}"
-    "/OUT:${TARGET_LOC}" ${LIB_LOCATIONS}
-    )
+  IF(CMAKE_LINKER MATCHES "lld-link")
+    # lld-link does not yet support making convenience libraries;
+    # use MSVC's linker. We assume link.exe is in our path.
+    EXECUTE_PROCESS(
+      COMMAND link.exe "/lib" "/NOLOGO" "${CMAKE_STATIC_LINKER_FLAGS}"
+      "/OUT:${TARGET_LOC}" ${LIB_LOCATIONS}
+      )
+  ELSE()
+    EXECUTE_PROCESS(
+      COMMAND ${CMAKE_LINKER} "/lib" "/NOLOGO" "${CMAKE_STATIC_LINKER_FLAGS}"
+      "/OUT:${TARGET_LOC}" ${LIB_LOCATIONS}
+      )
+  ENDIF()
 ELSEIF(APPLE)
   # libtool handles it for us
   EXECUTE_PROCESS(