Make zlib compatible with clang again

Include wmmintrin.h in configure test and crc_folding.c aid clang compilation.
Also add some missing forward declarations.

Resolves the following error messages in configure (gcc being symlinked to clang on default OS X install):

    gcc -O3 -DHAVE_HIDDEN -mpclmul ztest3051.c
    ztest3051.c:6:17: warning: implicit declaration of function '_mm_clmulepi64_si128' is invalid in C99 [-Wimplicit-function-declaration]
        __m128i c = _mm_clmulepi64_si128(a, b, 0x10);
                    ^
    ztest3051.c:6:13: error: initializing '__m128i' with an expression of incompatible type 'int'
        __m128i c = _mm_clmulepi64_si128(a, b, 0x10);
                ^   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    1 warning and 1 error generated.
    (exit code 1)
    Checking for PCLMULQDQ intrinsics ... No.

And the corresponding error in crc_folding.c