Refactor API macros into a new header, boost/system/api_config.hpp.
Prohibit user definition of API macros. Rationale: ensure all translation units use same definitions, cut number of environments that need to be tested.
[SVN r62313]
The symptom, which only occurs in some applications, is a crash due to a
dereference of a null pointer. The exact conditions under which the problem
occurs are not fully understood, so this fix is probably more paranoid than
necessary.
[SVN r41100]