Fix Visual Studio build.

Commit 8f469d97e2 introduced SSSE3 fast
paths that are gated by __SSE3__ macro checks and the <x86intrin.h>
header, neither of which exists in Visual Studio. This commit adds logic
for detecting SSSE3 compiler support that works for all compilers
supported by the open source release.

The commit also replaces the header with <tmmintrin.h>, which only
defines intrinsics supported by SSSE3 and below. This should help flag
any use of SIMD instructions that require more advanced SSE support, so
the uses can be gated by checks that also work in the open source
release.

Last, this commit requires C++11 support for the open source build. This is
needed by the alignas specifier, which was also introduced in commit
8f469d97e2.

cmake/config.h.in

@@ -55,6 +55,9 @@
 /* Define to 1 if you have the <windows.h> header file. */
 #cmakedefine HAVE_WINDOWS_H 1
 
+/* Define to 1 if you target processors with SSSE3+ and have <tmmintrin.h>. */
+#cmakedefine01 SNAPPY_HAVE_SSSE3
+
 /* Define to 1 if your processor stores words with the most significant byte
    first (like Motorola and SPARC, unlike Intel and VAX). */
 #cmakedefine SNAPPY_IS_BIG_ENDIAN 1