Changed COMPILING.md to use c++17

Changing the documentation COMPILING.md to reflect the change to C++17.
Also increased the minimum G++ version to 7 as it is the lowest version
with full C++17 support.

Author: Enrico Steffinlongo

COMPILING.md

@@ -1,6 +1,6 @@
 # What architecture?
 
-CPROVER now needs a C++11 compliant compiler and is known to work in the
+CPROVER now needs a C++17 compliant compiler and is known to work in the
 following environments:
 
 - Linux
@@ -152,7 +152,7 @@ We assume that you have a Debian/Ubuntu or Red Hat-like distribution.
    ```
    dnf install gcc gcc-c++ flex bison curl patch
    ```
-   Note that you need g++ version 5.0 or newer.
+   Note that you need g++ version 7.0 or newer.
 
    On Amazon Linux and similar distributions, do as root:
    ```