Bugfix duplicate symbol error in time_measure.hpp

[marksantaniello]

Free functions that are defined in header files must be qualified inline in order to avoid duplicate symbol errors at link time. Bugfix two cases:

• boost::sort::common::now()
• boost::sort::common::subtract_time()

[spreadsort]

I see a test failure:

...failed clang-linux.compile.c++ bin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-14-iso/optimization-speed/threading-multi/visibility-hidden/test_parallel_stable_sort.o...
...skipped <pbin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-14-iso/optimization-speed/threading-multi/visibility-hidden>test_parallel_stable_sort for lack of <pbin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-14-iso/optimization-speed/threading-multi/visibility-hidden>test_parallel_stable_sort.o...
...skipped <pbin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-14-iso/optimization-speed/threading-multi/visibility-hidden>test_parallel_stable_sort.run for lack of <pbin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-14-iso/optimization-speed/threading-multi/visibility-hidden>test_parallel_stable_sort...
clang-linux.compile.c++ bin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-1z-iso/optimization-speed/threading-multi/visibility-hidden/test_parallel_stable_sort.o
In file included from libs/sort/test/test_parallel_stable_sort.cpp:20:
In file included from ./boost/sort/parallel_stable_sort/parallel_stable_sort.hpp:23:
./boost/sort/sample_sort/sample_sort.hpp:376:25: error: 'this' cannot be explicitly captured when the capture default is '='
auto func = =, this
~~^~~~
1 error generated.

"clang++" -c -x c++ -std=c++1z -fvisibility-inlines-hidden -fPIC -m64 -pthread -O3 -fno-inline -Wall -g -fvisibility=hidden --target=x86_64-pc-linux -DBOOST_ALL_NO_LIB=1 -I"." -I".." -o "bin.v2/libs/sort/test/test_parallel_stable_sort.test/clang-linux-3.8/debug/cxxstd-1z-iso/optimization-speed/threading-multi/visibility-hidden/test_parallel_stable_sort.o" "libs/sort/test/test_parallel_stable_sort.cpp"

[marksantaniello]

Seems unrelated to this PR. I think it was caused by this commit, which added "this" in the lambda capture list.
d2b2cf6