CMake support as an alternative to AutoTools

This commit adds support anddocumentation for a CMake build.

Everything supported by the autotools build should be present except the following differences:

- The test suite is not present
- The DEBUG build adds more checks (and more warnings at build time that will be addressed eventually)
- Removes some header checks that seems not to be used in the autotools (like emmintrin.h, tmmintrin.h ...)
- __STDC_LIMIT_MACROS is not tested
- enable_asm flag is supported but there is no C+asm build test
- for 128-bit arithmetic is supported and can be enabled but there is no build test support
- clock_gettime, fegetenv, SecureZeroMemory tests have been removed as those functions are used regardless their presence in the code.

I hope this will work for most people out there. CMake could even be used to build on Windows platforms.

Feedbacks are welcome.

Besides, i have noticed that the latest HEAD built with autotools has a version of 0.4.2 but
the soversion is 4.3. Is it correct ? (the cmake build system uses SODIUM_LIBRARY_VERSION_MAJOR
and SODIUM_LIBRARY_VERSION_MINOR to set the soversion file extension)