Update setup-gcc.sh to recognize new gcc versions

From the current document on https://gcc.gnu.org/develop.html
```
Version	DEV-PHASE	When
5.0.0	(experimental) 	during active development of GCC 5 (stage 1 to 3)
5.0.1	(prerelease) 	during the stabilization period of GCC 5
6.0.0	(experimental) 	during active development of GCC 6 (stage 1 to 3)
5.1.0		for the first release from the GCC 5 branch
5.1.1		during development on the branch post the 5.1.0 release
5.2.0		for the second release from the GCC 5 branch
5.2.1		during development on the branch post the 5.2.0 release
6.0.1	(prerelease) 	during the stabilization period of GCC 6
```

It looks to me as though the major version is just the first digit, rather than the first two digits.  So we can expect that the difference between 5.1 and 5.2 should be no different to the difference between 4.8.1 and 4.8.2.  From an API point of view, they are compatible (don't require separate patches).

So from version 5 onwards, we don't need to know what the minor version is, as far as maintaining patches goes.

```
elif grep -q -E '^5\.[0-9]([^0-9]|$)' $d_gccsrc/gcc/BASE-VER; then
    gcc_ver=5
```
Don't need to worry about version 6, 7, ad infinitum... until later.