[thirdparty] fix boost build with clang from Xcode
[thirdparty] fix boost build with clang from Xcode

The clang version string in Xcode (see [1]) uses slightly different
format than clang built via MacPorts/Homebrew or clang on various
Linux flavors.  This patch adapts the build_boost() function to work
with clang from Xcode.

[1] https://gist.github.com/yamaya/2924292

Change-Id: I082af11af012cfb80a89ddb5ddf638100eb74574
Reviewed-on: http://gerrit.cloudera.org:8080/14089
Tested-by: Kudu Jenkins
Reviewed-by: Adar Dembo <866ca730bfc7f544e3ebce604ceeba4879f23df0@cloudera.com>

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