configure: remove workarounds for GCC < 4.8

They can't compile io.js anyway.

Additionally, it seems the 'gcc_version' variable is no longer used in config.gypi, perhaps it can be removed too? The 'clang' variable is now only used to enable [this monstrosity](https://github.com/iojs/io.js/blob/v0.12/common.gypi#L255), which was apparently copied from V8. However, V8 currently [enables](https://github.com/iojs/io.js/blob/v0.12/deps/v8/build/standalone.gypi#L119) it based only on the OS, which makes sense, since only Mac OS users are going to use XCode. Maybe io.js could do the same, eliminating the need to detect compilers?

On a less related note, why does io.js use -std=gnu++0x instead of -std=c++11? I've tried the latter and it builds just fine.