configure.ac (--enable-symbols-visibility-options): Add new option to
allow turning off compiler and platform specific options for symbols
visibility. Add default values description into configure options
explanations texts.

configure: Regenerate.

README: Document the new option.
timehelper.cxx (Time::gettimeofday): Fall back to different method of getting time
in the unlikely case where clock_gettime() fails.
README: Improve Markdown markup. Add notes about compilation and OpenBSD
and a section about contributions.