configure: Clang version check should use "based on" version for Apple Clang

[lilyball]

Apple Clang version numbering doesn't match the open source project. Typically it's a full major version ahead. This is why the configure script currently allows for 4.1* and 4.2*. However, this isn't particularly future-proof. The configure script should instead learn to use the "based on" version when present, because this describes what the open source version is. For example, the version string looks something like "Apple LLVM version 4.2 (clang-421.2.1) (based on LLVM 3.2svn)".

[lilyball]

Here's a new version of the sed command that should produce the "based on" version when the version string contains it: sed 's/.*\(version .*\)/\1/; s/.*based on \(LLVM .*\))/\1/'

[lilyball]

This was just handled via the pull request #6739