Make boost::coroutine optional

This is essentially because of a Debian bug, where no boost::coroutine
shared libraries are available.
8 jobs for master
Status Job ID Name Coverage
  Test
passed #482
amd64
job_clang_amd64

04:50:18

passed #486
amd64
job_clang_amd64_nosh

04:09:50

passed #484
amd64
job_clang_py2_amd64

04:09:26

failed #479
i386
job_gcc_386

04:50:34

passed #481
amd64
job_gcc_amd64

03:58:10

passed #485
amd64
job_gcc_amd64_nosh

03:15:07

failed #480
i386
job_gcc_py2_386

05:38:33

passed #483
amd64
job_gcc_py2_amd64

03:11:32

 
Name Stage Failure
failed
job_gcc_py2_386 Test There is an unknown failure, please try again
3 tests in 1 items.
3 passed and 0 failed.
Test passed.

Document: search_module
-----------------------
python2: symbol lookup error: /home/gitlab-runner/builds/c098b1de/1/count0/graph-tool/install/site-packages/graph_tool/search/libgraph_tool_search.so: undefined symbol: _ZN5boost10coroutines12stack_traits12default_sizeEv

ERROR: Build failed with: exit status 127
failed
job_gcc_386 Test There is an unknown failure, please try again
161 tests in 1 items.
161 passed and 0 failed.
Test passed.

Document: search_module
-----------------------
python3: symbol lookup error: /home/gitlab-runner/builds/c098b1de/0/count0/graph-tool/install/site-packages/graph_tool/search/libgraph_tool_search.so: undefined symbol: _ZN5boost10coroutines12stack_traits12default_sizeEv

ERROR: Build failed with: exit status 127