Windows: Added `/EHsc` to `CMAKE_CXX_FLAGS`.
Windows: Added `/EHsc` to `CMAKE_CXX_FLAGS`.

After upgrading Visual Studio to 15.5.2, warning C4530 started being
emitted for practically every file. Since we use the STL (which uses
exceptions), apparently we need to compile with `/EHsc`. This sets the
exception-handling model to catch C++ exceptions, and to assume that
functions declared as `extern "C"` never throw a C++ exception.

Review: https://reviews.apache.org/r/64701

asamerh4/mesos,asamerh4/mesos,chhsia0/mesos,reneploetz/mesos,gsantovena/mesos,asamerh4/mesos,reneploetz/mesos,gsantovena/mesos,jpeach/mesos,abudnik/mesos,chhsia0/mesos,kaysoky/mesos,chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,abudnik/mesos,abudnik/mesos,jpeach/mesos,Gilbert88/mesos,Gilbert88/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,asamerh4/mesos,gsantovena/mesos,chhsia0/mesos,andschwa/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,chhsia0/mesos,kaysoky/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,jpeach/mesos,Gilbert88/mesos,dforsyth/mesos,abudnik/mesos,dforsyth/mesos,andschwa/mesos,abudnik/mesos,jpeach/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,reneploetz/mesos,chhsia0/mesos,andschwa/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,asamerh4/mesos,jpeach/mesos,reneploetz/mesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,asamerh4/mesos,andschwa/mesos,jpeach/mesos,asamerh4/mesos,Gilbert88/mesos,Gilbert88/mesos,reneploetz/mesos,dforsyth/mesos,andschwa/mesos