Remove obsolete MSVC workarounds

The linker warning suppression targeted gflags, which Ceres no longer
uses, and _VARIADIC_MAX only affected the emulated variadic templates of
Visual Studio 2012, which cannot compile Ceres anymore. The comment on
the disabled DLL interface warning still referred to glog and gflags.

Drop both workarounds and describe why the warning remains disabled.

Change-Id: I5277d0e17020957c0d6c8596e653b9a6ee18d48d

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 337d9f4..4007bb1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -548,25 +548,15 @@
   # this warning may not be correct; needs investigation.
   # TODO(keir): Investigate these warnings in more detail.
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:/wd4244>)
-  # It's not possible to use STL types in DLL interfaces in a portable and
-  # reliable way. However, that's what happens with Google Log and Google Flags
-  # on Windows. MSVC gets upset about this and throws warnings that we can't do
-  # much about. The real solution is to link static versions of Google Log and
-  # Google Test, but that seems tricky on Windows. So, disable the warning.
+  # MSVC warns about exported classes with members whose types, such as those
+  # from the standard library, are not exported themselves. This cannot be
+  # avoided portably, so disable the warning.
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:/wd4251>)
 
   # Add bigobj flag otherwise the build would fail due to large object files
   # probably resulting from generated headers (like the fixed-size schur
   # specializations).
   add_compile_options($<$<COMPILE_LANGUAGE:CXX>:/bigobj>)
-
-  # Google Flags doesn't have their DLL import/export stuff set up correctly,
-  # which results in linker warnings. This is irrelevant for Ceres, so ignore
-  # the warnings.
-  set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} /ignore:4049")
-
-  # Tuple sizes of 10 are used by Gtest.
-  add_definitions("-D_VARIADIC_MAX=10")
 endif (MSVC)
 
 if (UNIX)