Be more consistent with other OSes. Try to avoid failing on unknown _XOPEN_SOURCE values.
3801 _XOPEN_SOURCE value is ignored if __XOPEN_SOURCE_EXTENDED is set

*** 234,248 ****
   * requirements, the appropriate feature test macros must be defined at
   * compile time. These are as follows. For more info, see standards(5).
   *
   * Feature Test Macro                                Specification
   * ------------------------------------------------  -------------
!  * _XOPEN_SOURCE                                         XPG3
!  * _XOPEN_SOURCE && _XOPEN_VERSION = 4                   XPG4
   * _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED = 1           XPG4v2
!  * _XOPEN_SOURCE = 500                                   XPG5
!  * _XOPEN_SOURCE = 600  (or POSIX_C_SOURCE=200112L)      XPG6
   *
   * In order to simplify the guards within the headers, the following
   * implementation private test macros have been created. Applications
   * must NOT use these private test macros as unexpected results will
   * occur.
--- 234,248 ----
   * requirements, the appropriate feature test macros must be defined at
   * compile time. These are as follows. For more info, see standards(5).
   *
   * Feature Test Macro                                Specification
   * ------------------------------------------------  -------------
!  * _XOPEN_SOURCE >= 600  (or POSIX_C_SOURCE>=200112L)    XPG6
!  * _XOPEN_SOURCE >= 500                                  XPG5
   * _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED = 1           XPG4v2
!  * _XOPEN_SOURCE && _XOPEN_VERSION = 4                   XPG4
!  * _XOPEN_SOURCE                                         XPG3
   *
   * In order to simplify the guards within the headers, the following
   * implementation private test macros have been created. Applications
   * must NOT use these private test macros as unexpected results will
   * occur.
*** 260,301 ****
   * _XPG4_2  X/Open CAE Specification, Issue 4, Version 2 (XPG4v2/UNIX 95/SUS)
   * _XPG5    X/Open CAE Specification, Issue 5 (XPG5/UNIX 98/SUSv2)
   * _XPG6    Open Group Technical Standard, Issue 6 (XPG6/UNIX 03/SUSv3)
   */
  
! /* X/Open Portability Guide, Issue 3 */
! #if defined(_XOPEN_SOURCE) && (_XOPEN_SOURCE - 0 < 500) && \
!         (_XOPEN_VERSION - 0 < 4) && !defined(_XOPEN_SOURCE_EXTENDED)
! #define _XPG3
! /* X/Open CAE Specification, Issue 4 */
! #elif   (defined(_XOPEN_SOURCE) && _XOPEN_VERSION - 0 == 4)
! #define _XPG4
! #define _XPG3
! /* X/Open CAE Specification, Issue 4, Version 2 */
! #elif (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE_EXTENDED - 0 == 1)
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
  /* X/Open CAE Specification, Issue 5 */
! #elif   (_XOPEN_SOURCE - 0 == 500)
  #define _XPG5
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
  #undef  _POSIX_C_SOURCE
  #define _POSIX_C_SOURCE                 199506L
! /* Open Group Technical Standard , Issue 6 */
! #elif   (_XOPEN_SOURCE - 0 == 600) || (_POSIX_C_SOURCE - 0 == 200112L)
! #define _XPG6
! #define _XPG5
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
! #undef  _POSIX_C_SOURCE
! #define _POSIX_C_SOURCE                 200112L
! #undef  _XOPEN_SOURCE
! #define _XOPEN_SOURCE                   600
  #endif
  
  /*
   * _XOPEN_VERSION is defined by the X/Open specifications and is not
   * normally defined by the application, except in the case of an XPG4
--- 260,300 ----
   * _XPG4_2  X/Open CAE Specification, Issue 4, Version 2 (XPG4v2/UNIX 95/SUS)
   * _XPG5    X/Open CAE Specification, Issue 5 (XPG5/UNIX 98/SUSv2)
   * _XPG6    Open Group Technical Standard, Issue 6 (XPG6/UNIX 03/SUSv3)
   */
  
! /* Open Group Technical Standard , Issue 6 */
! #if     (_XOPEN_SOURCE - 0 >= 600) || (_POSIX_C_SOURCE - 0 >= 200112L)
! #define _XPG6
! #define _XPG5
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
+ #undef  _POSIX_C_SOURCE
+ #define _POSIX_C_SOURCE                 200112L
+ #undef  _XOPEN_SOURCE
+ #define _XOPEN_SOURCE                   600
  /* X/Open CAE Specification, Issue 5 */
! #elif   (_XOPEN_SOURCE - 0 >= 500)
  #define _XPG5
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
  #undef  _POSIX_C_SOURCE
  #define _POSIX_C_SOURCE                 199506L
! /* X/Open CAE Specification, Issue 4, Version 2 */
! #elif (defined(_XOPEN_SOURCE) && _XOPEN_SOURCE_EXTENDED - 0 == 1)
  #define _XPG4_2
  #define _XPG4
  #define _XPG3
! /* X/Open CAE Specification, Issue 4 */
! #elif   (defined(_XOPEN_SOURCE) && _XOPEN_VERSION - 0 == 4)
! #define _XPG4
! #define _XPG3
! /* X/Open Portability Guide, Issue 3 */
! #elif defined(_XOPEN_SOURCE) && (_XOPEN_VERSION - 0 < 4)
! #define _XPG3
  #endif
  
  /*
   * _XOPEN_VERSION is defined by the X/Open specifications and is not
   * normally defined by the application, except in the case of an XPG4