x86-64 SIMD build fixes (#20)

x86-64 SIMD build fixes (#20)

* x86-64 SIMD build fixes

configure.ac was modified to detect g++ >=5 and clang++ >=7. Additionally
some script malfunctions on FreeBSD were corrected.

The get_checksum1() code has been modified to fix clang and g++ 10
compilation.

This version of the code and configure.ac has been tested on:

Ubuntu 16 - gcc 7.3.0, clang 6.0.0
Debian 10 - gcc 5.4.0, 6.4.0, 7.2.0, 8.4.0, 9.2.1, 10.0.1, clang 5.0.2,
6.0.1, 7.0.1, 8.0.0, 9.0.0, 10.0.0
ArchLinux 20200605 - gcc 10.1.0, clang 10.0.0
FreeBSD 12.1 - gcc 9.3.0, clang 8.0.1

It is unknown if it will work on gcc 5.0-5.3, but the script currently
allows it.