based on the 'visibility' attribute for GNUC and __global for Sun
compilers, taking also in account __declspec function decoration
for Win32 and Symbian DLL's.
Introducing configure options --enable-hidden-symbols and
--disable-hidden-symbols following libcurl's naming.
option to specify dis(activation) of compiler optimizations.
If option is specified, it will be honored independant of the
--(dis|en)able-debug option.
option to specify dis(activation) of picky compiler warnings.
If option is specified, it will be honored independant of the
--(dis|en)able-debug option.
If option is not specified, it will follow --(dis|en)able-debug
setting, whose default is disabled if not specified.