Link boost statically only when requested

[MarkusTeufelberger]

Fixes #2261

[ripplelabs-jenkins]

Jenkins Build Summary

Built from this commit

Built at 20171202 - 21:47:35

Test Results

Build Type	Result	Status
Author Check	MarkusTeufelberger is not a collaborator!	FAIL 🔴

[codecov-io]

Codecov Report

Merging #2291 into develop will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #2291      +/-   ##
===========================================
- Coverage    70.96%   70.89%   -0.08%     
===========================================
  Files          691      691              
  Lines        51663    51120     -543     
===========================================
- Hits         36661    36239     -422     
+ Misses       15002    14881     -121

Impacted Files	Coverage Δ
src/ripple/beast/clock/chrono_util.h	82.6% <0%> (-8.7%)	⬇️
src/ripple/basics/DecayingSample.h	77.77% <0%> (-8.34%)	⬇️
src/ripple/app/ledger/PendingSaves.h	93.54% <0%> (-6.46%)	⬇️
src/ripple/app/ledger/TransactionStateSF.cpp	50% <0%> (-3.85%)	⬇️
src/ripple/core/SociDB.h	60% <0%> (-3.64%)	⬇️
src/ripple/app/ledger/AcceptedLedger.cpp	66.66% <0%> (-2.09%)	⬇️
src/ripple/beast/utility/WrappedSink.h	54.54% <0%> (-1.98%)	⬇️
src/ripple/nodestore/impl/DatabaseRotatingImp.h	60.52% <0%> (-1.98%)	⬇️
src/ripple/conditions/impl/error.cpp	43.24% <0%> (-1.5%)	⬇️
src/ripple/server/impl/Port.cpp	68.9% <0%> (-1.41%)	⬇️
... and 204 more

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[JoeLoser]

Any interest in getting this through still @bachase? I saw this PR had no reviewers assigned to it and it's a small, good change.

[bachase]

@mellery451 and @ximinez feel free to retag, but your cmake preferences are highly weighted!

[mellery451]

👍

[mellery451]

(built using private branch)

Jenkins Build Summary

Built from this commit

Built at 20171220 - 19:02:52

Test Results

Build Type	Result	Status
coverage	970 cases, 0 failed, t: 610s	PASS ✅
clang.debug.unity	970 cases, 0 failed, t: 382s	PASS ✅
gcc.debug.unity	970 cases, 0 failed, t: 434s	PASS ✅
clang.debug.nounity	968 cases, 0 failed, t: 589s	PASS ✅
clang.release.unity	969 cases, 0 failed, t: 469s	PASS ✅
gcc.debug.nounity	968 cases, 0 failed, t: 691s	PASS ✅
gcc.release.unity	969 cases, 0 failed, t: 499s	PASS ✅

[seelabs]

In 0.90.0-b3