Use new MSVC preprocessor for VS 17 builds

From the documentation[1]:

| We're updating the Microsoft C++ preprocessor to improve standards
| conformance, fix longstanding bugs, and change some behaviors that
| are officially undefined. We've also added new diagnostics to warn
| on errors in macro definitions.

Thus it appears to be sensible to switch to the new preprocessor for
all VS17 builds.  Note that although the new preprocessor is available
as of VS16.5 (i.e. `version >= 1925`), we don't want to potentially
break older builds, and as such use the new preprocessor only for VS17
builds.  Users who wish to use the new preprocessor with VS16, can
still `set CFLAGS=/Zc:preprocessor` before running `buildconf`/`phpize`.

[1] <https://learn.microsoft.com/en-us/cpp/preprocessor/preprocessor-experimental-overview?view=msvc-170>

Closes GH-15201.

Author: cmb69

UPGRADING.INTERNALS

@@ -224,6 +224,7 @@ PHP 8.4 INTERNALS UPGRADE NOTES
      the openssl extension is available either as shared or built statically.
    - Added configure option --enable-phpdbg-debug to build phpdbg in debug mode.
    - The win32/build/libs_version.txt file has been removed.
+   - MSVC builds now use the new preprocessor (/Zc:preprocessor).
 
 ========================
 3. Module changes

win32/build/confutils.js

@@ -3313,6 +3313,10 @@ function toolset_setup_common_cflags()
 			ADD_FLAG("CFLAGS", "/d2FuncCache1");
 		}
 
+		if (VCVERS >= 1930) {
+			ADD_FLAG("CFLAGS", "/Zc:preprocessor");
+		}
+
 		ADD_FLAG("CFLAGS", "/Zc:wchar_t");
 	} else if (CLANG_TOOLSET) {
 		if (TARGET_ARCH == 'x86') {