Add test to verify headers are self-contained

[mloskot]

For each header, a translation unit is generated along with corresponding compile target.
Currently, only boost/gil/*.hpp and boost/gil/io/* headres are included.

On CI services, compile self_contained_headers targets only if TEST_HEADERS environment variable is set. This is to avoid build timeouts due to CI services limits.
When running b2 locally, the tests are compiled by default.

---

NOTE: The self-contained headers tests are built by default. There will be compilation errors due to actual 'broken' headers! Obviously, those need to be fixed via dedicated PRs/commits.

References

• Add #include directives with missing headers #148

Tasklist

• Review
• Adjust for comments
• All CI builds and checks have passed

[stefanseefeld]

I'm not sure to fully understand the (Jamfile) logic, but conceptually this looks good to me. So, trusting that the CI will catch any issues, I'm going to approve this.

[mloskot]

Feel free to ask specific questions about that script. I'll do my best to explain.

[mloskot]

A sample of errors caught by the two CI-s:

• https://ci.appveyor.com/project/stefanseefeld/gil/build/1.0.483-develop/job/1nf3rwqa5qn3nwc1

C:\projects\boost\boost/gil/io/bit_operations.hpp(28): 
    error C2061: syntax error: identifier 'byte_t'

• https://travis-ci.org/boostorg/gil/jobs/436780478

./boost/gil/premultiply.hpp: In member function ‘void boost::gil::premultiply::operator()(const SrcP&, DstP&) const’:
./boost/gil/premultiply.hpp:59:3: 
    error: ‘for_each’ is not a member of ‘boost::mpl’
        mpl::for_each<src_colour_channels>(channel_premultiply<SrcP, DstP>(src, dst));
       ^

[mloskot]

The new test require more time from the CI jobs, while the allowance is limited, eg.
https://travis-ci.org/boostorg/gil/builds/437264162

My plan is to:

• Improve Jamfile to make the self_contained_headers test built on demand only via explicit b2 <target> request or dedicated environment variable
• Update CI builds to run the headers test only during single job, eg. GCC 5

[mloskot]

Done in 9dc67ea

On CI services, compile self_contained_headers targets only if TEST_HEADERS environment variable is set. This is to avoid build timeouts due to CI services limits. When running b2 locally, the tests are compiled by default.