CMake: Do not enable LTO by default for MSVC

LTO is supposed to be enabled by default for VS Release builds
according to the VS prop files but a build log from JMC reveals
that /GL and /LTCG are not actually passed to cl.exe/link.exe
for some reason...

LTO also leads to *extremely* and unacceptably slow build times
when using link.exe, so let's disable it by default to actually
match the project files.

CMakeLists.txt

@@ -332,11 +332,16 @@ if(CMAKE_SYSTEM_NAME MATCHES "Darwin")
 endif()
 
 if(ENABLE_LTO)
+  if(CMAKE_C_COMPILER_ID MATCHES "MSVC")
+    add_compile_options(/GL)
+    string(APPEND CMAKE_EXE_LINKER_FLAGS " /LTCG")
+  else()
     check_and_add_flag(LTO -flto)
     if(CMAKE_CXX_COMPILER_ID STREQUAL GNU)
       set(CMAKE_AR gcc-ar)
       set(CMAKE_RANLIB gcc-ranlib)
     endif()
+  endif()
 endif()
 
 if(UNIX AND LINUX_LOCAL_DEV)
@@ -362,11 +367,6 @@ if(NOT CMAKE_BUILD_TYPE)
       "Build type (Release/Debug/RelWithDebInfo/MinSizeRel)" FORCE)
 endif()
 
-if(CMAKE_BUILD_TYPE MATCHES "Release|RelWithDebInfo" AND CMAKE_C_COMPILER_ID MATCHES "MSVC")
-  add_compile_options(/GL)
-  string(APPEND CMAKE_EXE_LINKER_FLAGS " /LTCG")
-endif()
-
 if(ENABLE_GPROF)
   check_and_add_flag(HAVE_PG -pg)
   if(NOT FLAG_C_HAVE_PG)