Update README.md minimal compilers

Update the minimal compilers in documentation. As `std=c++17` is used the minimal got quite bumped from last update. For gcc 7.x would seem possibly enough, but some of the dependencies (eg. libpqxx) need 8.x at least, so setting that as documented minimum.

README.md

@@ -58,7 +58,7 @@ To build on Mac and Linux just type `make`. On FreeBSD and OpenBSD `gmake` (GNU
    - Xcode command line tools for macOS 10.13 or newer are required.
    - Rust for x86_64 and ARM64 targets *if SSO is enabled in the build*.
  - **Linux**
-   - The minimum compiler versions required are GCC/G++ 4.9.3 or CLANG/CLANG++ 3.4.2. (Install `clang` on CentOS 7 as G++ is too old.)
+   - The minimum compiler versions required are GCC/G++ 8.x or CLANG/CLANG++ 5.x.
    - Linux makefiles automatically detect and prefer clang/clang++ if present as it produces smaller and slightly faster binaries in most cases. You can override by supplying CC and CXX variables on the make command line.
    - Rust for x86_64 and ARM64 targets *if SSO is enabled in the build*.
  - **Windows**