Fix configure script to work with Apple Clang 11
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Xcode 11's clang doesn't match our regex:
$ clang --version
Apple clang version 11.0.0 (clang-1100.0.20.17)
Target: x86_64-apple-darwin19.0.0
Thread model: posix
InstalledDir: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
I've updated the regex in the configure script so that it will try to match the newer one if the old one fails.