Refactor PHP/SystemExtensions module

- Obsolete checks for some old Solaris systems where defining
  __EXTENSIONS__ breaks system headers has been removed. Building
  software on such systems will bring also other issues and in PHP's
  case most definitely such system can't be used anymore. This issue is
  obsolete.
- _DARWIN_C_SOURCE defined conditionally only for Darwin targets.
- Updated docs.

Author: petk