[Backport] Revert undefining some defines (#103)

- this has been introduced in commontk#89 and breaks builds including "pythread.h"
- fixes #103

(cherry picked from commit MeVisLab/pythonqt@30ca585)

Author: mrbean-bremen

src/PythonQtPythonInclude.h

@@ -33,19 +33,6 @@
 #ifndef __PythonQtPythonInclude_h
 #define __PythonQtPythonInclude_h
 
-// Undefine macros that Python.h defines to avoid redefinition warning.
-#ifdef _POSIX_C_SOURCE
-#  undef _POSIX_C_SOURCE
-#endif
-
-#ifdef _POSIX_THREADS
-#  undef _POSIX_THREADS
-#endif
-
-#ifdef _XOPEN_SOURCE
-#  undef _XOPEN_SOURCE
-#endif
-
 // Undefine Qt keywords that conflict with Python headers
 #ifdef slots
 #undef slots