[MacPorts] #55926: Add C++11 variant to Boost

[MacPorts]

#55926: Add C++11 variant to Boost
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 Reporter:  michaelld               |      Owner:
     Type:  enhancement             |     Status:  new
 Priority:  Normal                  |  Milestone:
Component:  ports                   |    Version:
 Keywords:  boost, c++11, haspatch  |       Port:  boost
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 This ticket is a proposal to add +cxx11 variant to Boost to build Boost
 using -std=gnu++11 with a c++11 compliant compiler (via the cxx11 1.1
 PortGroup and appending to configure.cxxflags), and make it a default for
 Mac OS X <= 10.8.

 I've tested this change (+cxx11) on 10.12 and 10.8, and this small change
 works nicely. A benefit especially for <= 10.8 is that any port using the
 cxx11 1.1 PortGroup with boost as a dependency will work out of the box,
 instead of having to manually select a valid compiler (for Boost and/or
 the port).

 In my testing, compiling Boost using c++11 does not mean that any port
 using this Boost must also be compiled using c++11. Further, Boost +cxx11
 seems to be a drop-in replacement for Boost -cxx11.

 I'd even go so far as to say that Boost should -always- be compiled using
 c++11, since doing so will be more robust for building ports on both older
 and newer OSs.

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Ticket URL: <https://trac.macports.org/ticket/55926>
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