...as clang-cl doesn't support the /clr switch.

* In configure.ac, capture the MSCV version (that would be used if CC hadn't
  been overridden to use clang-cl) into MSVC_CXX.

* The logic which flags to pass into gb_CObject__command_pattern is coded into
  the platform-agnostic LinkTarget.mk, so it's too late to try and filter all
  relevant flags in com_GCC_class.mk, depending on whether a given .cxx file is
  a normal one built with the normal $CXX or a special /clr one built with
  $MSVC_CXX.  Thus, a new CxxClrObject class had to be introduced that captures
  this information early.

* When building with clang-cl, the generated config_host/config_*.h files
  contain values suitable for clang-cl, but not for MSVC.  But the .cxx files
  compiled with MSVC happen to include config_global.h, and would fail.  Hack
  around that problem for now by introducing a hard-coded, minimal
  solenv/clang-cl/config_global.h that is found first when buliding such a
  CxxClrObject.  Needs cleaning-up properly.

Change-Id: Iff8aac51c0b4fa906b14503c692640dda0996d33
Reviewed-on: https://gerrit.libreoffice.org/34509Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>