Clang on Windows and GNU ABI

[blapie]

It looks like the build system is special-cased for clang on windows (using SLEEF_CLANG_ON_WINDOWS variable).
Most of these cases make sense, but in Configure.cmake we do not able GNUABI if SLEEF_CLANG_ON_WINDOWS and nothing seems to justify/explain it.

if (COMPILER_SUPPORTS_WEAK_ALIASES AND
    NOT CMAKE_SYSTEM_PROCESSOR MATCHES "arm" AND
    NOT CMAKE_SYSTEM_PROCESSOR MATCHES "^(powerpc|ppc)64" AND
    NOT (SLEEF_CLANG_ON_WINDOWS) AND
    NOT MINGW AND SLEEF_BUILD_GNUABI_LIBS)
  set(ENABLE_GNUABI ${COMPILER_SUPPORTS_WEAK_ALIASES})

What is wrong with Clang on Windows and GNU ABI?

Remember at the time windows also implied x86, but soon it won't.