clang-cl does not like it if we try to check values of __GNUC__ without first
checking whether it is defined, so fix things up here.  clang-cl defines
_MSC_VER and __clang__ but not __GNUC__.

clang-cl follows some of the compiler flags that are used for GCC, but not all
them, so make sure that we don't try to use the compiler flags that it does not
understand.

Since the Visual Studio headers and libraries that clang-cl uses supported
SSE 4.1 for a very long while, even before the inception of clang-cl, always
pass in '-msse4.1' in the cflags when we are building with clang-cl (and it is
needed, otherwise the build fails).

We can actually turn off the deprecation warnings from the Visual Studio
headers that clang-cl uses, so let's do that.

...for SSE and ARM NEON intrinsics, since the __extension__ stuff is supported
on clang-cl.  Update the error messages accordingly.