Use correct __cplusplus value with MSVC

...now that /Zc:__cplusplus is available in Visual Studio 2017 version 15.7
(see <https://blogs.msdn.microsoft.com/vcblog/2018/04/09/
msvc-now-correctly-reports-__cplusplus/>).

Some external projects might run into issues when picking up /Zc:__cplusplus
directly with $(CXXFLAGS_CXX11) or indirectly via $(gb_CXXFLAGS) now, but that
appears not to be the case.

Some obsolete MSVC-specific __cplusplus checks can be removed now.  (The ones in
external/libebook/libebook-msvc.patch.1 pick up /Zc:__cplusplus via
$(gb_CXXFLAGS) in external/libebook/ExternalProject_libebook.mk.)

Change-Id: Idc6849a0000ea424522f30f61caba112fae25d40
Reviewed-on: https://gerrit.libreoffice.org/64755
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

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