Removed all configurations that explicitly test or set the c++ standard
flags. The only place the standard is now configured is at the top of
the main `CMakeLists.txt` file, which can easily be updated (e.g. if
we decide to move to c++14+). This can also be set via command-line using
```
> cmake -DCMAKE_CXX_STANDARD 14
```
Kept the `EIGEN_TEST_CXX11` flag for now - that still controls whether to
build/run the `cxx11_*` tests. We will likely end up renaming these
tests and removing the `CXX11` subfolder.
The macro `__cplusplus` is not defined correctly in MSVC unless building
with the the `/Zc:__cplusplus` flag. Instead, it defines `_MSVC_LANG` to the
specified c++ standard version number.
Here we introduce `EIGEN_CPLUSPLUS` which will contain the c++ version
number both for MSVC and otherwise. This simplifies checks for supported
features.
Also replaced most instances of standard version checking via `__cplusplus`
with the existing `EIGEN_COMP_CXXVER` macro for better clarity.
Fixes: #2170
Ancient versions of CMake required else(), endif(), and similar block
termination commands to have arguments matching the command starting the block.
This is no longer the preferred style.
After yet another question on the forum, I decided to write something on this
common issue. Now we just need to link to this and get people to read it.
Thanks to mattb on the forum for some links. Caveat: IANALL (I am not a
language lawyer).
* change Sparse* specialization's signatures from <..., int Backend> to <..., typename Backend>. Update SparseExtra accordingly to use structs instead of the SparseBackend enum.
* add SparseLDLT Cholmod specialization
* for Cholmod and UmfPack, SparseLU, SparseLLT and SparseLDLT now use ei_solve_retval and have the new solve() method (to be closer to the 3.0 API).
* fix doc
- make the beginning more precise
- make the first example be a full selfcontained compiled example, no need for all the others, but having the first one doesn't hurt.
