Update installer's default/minimum GCC version to 8.2.0

It's still possible to override this default/minimum using
the installer flags --with-fortran, --with-c,  and --with-cxx.

Author: Damian Rouson

prerequisites/build-functions/set_or_print_default_version.sh

@@ -27,7 +27,7 @@ set_or_print_default_version()
   # See http://stackoverflow.com/questions/1494178/how-to-define-hash-tables-in-bash
   package_version=(
     "cmake:3.4.0"
-    "gcc:7.3.0"
+    "gcc:8.2.0"
     "mpich:3.2"
     "wget:1.16.3"
     "flex:2.6.0"