Simply matching "0 Warning" may match "10 Warning(s)". Instead remove
the entire ` 0 Warning(s)` content from the MSBuild output before
searching it for warnings.
Issue: #16942
The change in commit v3.9.0-rc1~6^2~1 (C++ feature checks: check output
for '[Ww]arning', 2017-06-03) accidentally matches `0 Warning(s)` in the
output and always thinks a warning exists, thus failing all checks in
Visual Studio builds.
Fixes: #16942
When using `CMake_NO_CXX_STANDARD` a user may still provide `-std=c++11`
or similar flags in `CMAKE_CXX_FLAGS`, so we should still check for the
available C++ features.
Turn the feature check for cxx11_unordered_map into a function such that
we can use it for other features as well. Drop the 11 suffix, as we may
want to check features from other standards.
Brad King