Clarify target C++ language standard

[Crzyrndm]

@tfussell Is this library targeting C++11 or C++14? The readme compile command and repo description state 14, but the cmake files are set to 11 (https://github.com/tfussell/xlnt/blob/master/source/CMakeLists.txt#L7).

[Crzyrndm]

I've been thinking about this and I can't find a significant benefit to this library targeting 14. The major bonus in my experience is the dramatically improved usability of constexpr (which doesn't help much in file IO).

Locations where c++14 is mentioned (could be more external)

Github

• https://github.com/tfussell/xlnt -> repo description

Repository (git grep c++14 (ignoring third party))

• README.md:// compile with -std=c++14 -Ixlnt/include -lxlnt
• docs/introduction/Examples.md:g++ -std=c++14 -lxlnt process.cpp -o process
• docs/introduction/Examples.md:g++ -std=c++14 -lxlnt process.cpp -o process
• docs/xlnt.3://\ compile\ with\ -std=c++14\ -Ixlnt/include\ -lxlnt
• docs/xlnt.3:g++\ -std=c++14\ -lxlnt\ process.cpp\ -o\ process
• docs/xlnt.3:g++\ -std=c++14\ -lxlnt\ process.cpp\ -o\ process

External

• https://github.com/Microsoft/vcpkg/blob/master/ports/xlnt/CONTROL -> vcpkg description

[tfussell]

That's very thorough! I don't want the library to "feel" outdated, but I agree that C++11 is probably a good target for the time being. Some users were reporting problems with C++14 features so I added C++11 workaround and only halfway updated docs. Once more distros support C++14+ with their built in compilers we can move to a later standard. I'll go through and update docs/comments/etc in the noted places.