Cleanup CMake and Documentation

[lrineau]

Summary of Changes

• pass a few CMake message from STATUS to VERBOSE, so that they are no longer displayed by default
• remove a few lines of deprecated CMake code
• remove mentions of CGAL/compiler_config.h and workaround macros in the CGAL documentation

Release Management

• Affected package(s): CGAL
• License and copyright ownership: no change

[albert-github]

Overnight documentation build gave:

/home/cgal-testsuite/cgal_doc_build/CGAL-6.2-Ic-81/doc/Documentation/Developer_manual/Chapter_portability.txt:14: warning: unable to resolve reference to 'secworkaround_flags' for \ref command
/home/cgal-testsuite/cgal_doc_build/CGAL-6.2-Ic-81/doc/Documentation/Developer_manual/Chapter_portability.txt:29: warning: unable to resolve reference to 'secworkaround_flags' for \ref command

the mentioned section (also in this file) has been removed so also should be done with these references

[sloriot]

Successfully tested in CGAL-6.2-Ic-82 (and doc locally fixed and checked)