Explicitly drop support for old clang versions on Windows

[cmb69]

This ensures compatibility with the typedef redefinitions we use, and makes it clear during the configure stage that old compiler versions are not supported.

See https://releases.llvm.org/ regarding the release history of clang.

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Note that clang 3.1.0 added support for C11, and that may include support for typedef redefinitions, but I'm not sure about that. And the oldest clang I've managed to install on my machine is 4.0.0. I did a minimal PHP build successfully, so I'm reasonable sure that the typedef redifinition won't cause an issue as of clang 4.0.0. And frankly, I don't see much point in claiming support for even older clang versions, while clang 4.0.0 has been released on 2017-03-13, and I'm not even sure that newer clang versions are fully supported with PHP on Windows.

[cmb69]

Note that in combination with PR #15403, this ensures compatibility with master (at least basic compatibility). There is, however, a third "supported" toolset on Windows, namely ICC. I have never used it, so I have no idea whether it is actually still useable, but I don't really mind sticking with what we have for ICC (and possibly let users report issues).

[Girgias]

LGTM

[cmb69]

I'm going to commit this, since attempting to support much older versions would cause more issues. E.g. there is

php-src/Zend/zend_portability.h

Lines 309 to 310 in a6b7f13

	#elif defined(_MSC_VER) && _MSC_VER >= 1800 && !defined(__clang__)
	# define ZEND_FASTCALL __vectorcall

So ZEND_FASTCALL for x64 builds is __vectorcall for MSVC, but __cdecl for clang. Of course, this is bad for interoperability, and likely causes linker issues (especially for clang < 13, which may fall back to MSVC compilation). Note that the clang exclusion had been commit on 2015-07-19, so targeted clang <= 3.6.2, while __vectorcall support has been added to clang as of 3.6.0. Maybe back then there have been glitches, or only an older clang version had been tested (clang 3.5.2 had only been released a few months prior to this commit).