[CMake] Compilation Error: Modern Compiler + Old STL

[Mathieu Garaud]

Hello,

While compiling cmake3.13.4 using pkgsrc I had an error because I'm using a
new version of clang7.0.1 witth an old version of libstdc++5.4.0.

I propose to change the preprocess tests using __cplusplus >= xxx  in
Source/cmAlgorithms.h by their appropriate define like you do for
unique_ptr using: CMake_HAVE_CXX_UNIQUE_PTR.

To make it work you need to add the extra cm_check_cxx_feature
in Source/Checks/cm_cxx_features.cmake and the test files in
Source/Checks: cm_cxx_size_t.cxx, cm_cxx_cbegin.cxx and cm_cxx_cend.cxx.

I attached the files I modified as example it won't take more than 15min to
diff, integrate and commit I promise. What do you think?

Best,

Mathieu
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