configure: set WEBP_HAVE_AVX2 when available

this is used to set WEBP_USE_AVX2 in files where the build flag won't be
used, i.e., dsp/enc.c, which enables VP8EncDspInitAVX2() to be called

Change-Id: I362f4ba39ca40d3e07a081292d5f743c649d9d7f
enc_sse2: prevent signed int overflow

_mm_movemask_epi8 returns a 16-bit mask; << 16 can overflow a signed
int.

Change-Id: Ia0bb0804fe548fb9b0edb3695e82727506066cda
VP8RandomBits2: prevent signed int overflow

'diff' at its largest may be INT_MAX; << 1 of anything at or above
1 << 30 will overflow.

Change-Id: Idb2b5a9b55acc2f6d5e32be8baaebee3f89919ad
Merge "VP8RandomBits2: prevent signed int overflow"
Merge "configure: set WEBP_HAVE_AVX2 when available"