…and flags for SIMD with MSVC

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

…4266)

This should address the following 2 issues when using MSVC:
- MSVC does not set defines like `__SSE2__` or `__SSE4_2__` etc. we must
do that ourselves
- MSVC uses the `/arch` flag differently than other compilers and it
should be set only once

The ramifications of not having those defines set basically means that
OIIO won't notice any SIMD is possible.

For MSVC the /arch flag only controls which instruction set the compiler
may use when generating code "on its own". For example, the x64 compiler
by default will use SSE2 whenever possible when its generating code but
when using `/arch:AVX` it will produce AVX code as it sees fit. NOTE:
case matters here too! "avx" does not work but "AVX" does.

However, if the source code uses intrinsics, MSVC will happily use those
instead. In fact it's possible to use `avx512` intrinsics without
specifying any /arch flag at all and it will happily generate
corresponding avx512 code. This is much different than say GCC where, in
order to use intrinsics at all, you must specify the corresponding flags
allowing you to do so for the arch in question.

Fixes #4265

## Tests

Existing simd_test.cpp should be sufficient I think. In addition to
those, I've verified the below locally:

Currently: when using `-DUSE_SIMD="sse4.2,avx2"` to build, we get the
wrong `build:simd` flags:
`sse2`

After: using the same flags as above:
`sse2,sse3,ssse3,sse41,sse42,avx,avx2`

---------

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

…cademySoftwareFoundation#4266)

This should address the following 2 issues when using MSVC:
- MSVC does not set defines like `__SSE2__` or `__SSE4_2__` etc. we must
do that ourselves
- MSVC uses the `/arch` flag differently than other compilers and it
should be set only once

The ramifications of not having those defines set basically means that
OIIO won't notice any SIMD is possible.

For MSVC the /arch flag only controls which instruction set the compiler
may use when generating code "on its own". For example, the x64 compiler
by default will use SSE2 whenever possible when its generating code but
when using `/arch:AVX` it will produce AVX code as it sees fit. NOTE:
case matters here too! "avx" does not work but "AVX" does.

However, if the source code uses intrinsics, MSVC will happily use those
instead. In fact it's possible to use `avx512` intrinsics without
specifying any /arch flag at all and it will happily generate
corresponding avx512 code. This is much different than say GCC where, in
order to use intrinsics at all, you must specify the corresponding flags
allowing you to do so for the arch in question.

Fixes AcademySoftwareFoundation#4265

## Tests

Existing simd_test.cpp should be sufficient I think. In addition to
those, I've verified the below locally:

Currently: when using `-DUSE_SIMD="sse4.2,avx2"` to build, we get the
wrong `build:simd` flags:
`sse2`

After: using the same flags as above:
`sse2,sse3,ssse3,sse41,sse42,avx,avx2`

---------

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>

…cademySoftwareFoundation#4266)

This should address the following 2 issues when using MSVC:
- MSVC does not set defines like `__SSE2__` or `__SSE4_2__` etc. we must
do that ourselves
- MSVC uses the `/arch` flag differently than other compilers and it
should be set only once

The ramifications of not having those defines set basically means that
OIIO won't notice any SIMD is possible.

For MSVC the /arch flag only controls which instruction set the compiler
may use when generating code "on its own". For example, the x64 compiler
by default will use SSE2 whenever possible when its generating code but
when using `/arch:AVX` it will produce AVX code as it sees fit. NOTE:
case matters here too! "avx" does not work but "AVX" does.

However, if the source code uses intrinsics, MSVC will happily use those
instead. In fact it's possible to use `avx512` intrinsics without
specifying any /arch flag at all and it will happily generate
corresponding avx512 code. This is much different than say GCC where, in
order to use intrinsics at all, you must specify the corresponding flags
allowing you to do so for the arch in question.

Fixes AcademySoftwareFoundation#4265

Existing simd_test.cpp should be sufficient I think. In addition to
those, I've verified the below locally:

Currently: when using `-DUSE_SIMD="sse4.2,avx2"` to build, we get the
wrong `build:simd` flags:
`sse2`

After: using the same flags as above:
`sse2,sse3,ssse3,sse41,sse42,avx,avx2`

---------

Signed-off-by: Jesse Yurkovich <jesse.y@gmail.com>