[fix] Add patch to keep _POSIX_THREADS value.

In UNIX platform since python 3.7, _POSIX_THREADS need to be defined. If
it's not the case the native threads type is unknown and a build error
is returned from pythread.h.

Projects/patches/pythonqt-fix-native-threads.patch

+From 1ccbae62bb5e704c61462d8ce49b43fa6ad0badd Mon Sep 17 00:00:00 2001
+From: Jerome Dias <jerome.dias@kitware.com>
+Date: Mon, 26 Oct 2020 16:24:44 +0100
+Subject: [PATCH] [fix] disable _POSIX_THREADS unset.
+
+In unix platform since python 3.7, _POSIX_THREADS need to be defined. If
+it's not the case the native threads type is unknown and a build error
+is returned in pythread.h.
+---
+ src/PythonQtPythonInclude.h | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/src/PythonQtPythonInclude.h b/src/PythonQtPythonInclude.h
+index be8d0e0..dc9c7f1 100644
+--- a/src/PythonQtPythonInclude.h
++++ b/src/PythonQtPythonInclude.h
+@@ -35,7 +35,7 @@
+ 
+ // Undefine macros that Python.h defines to avoid redefinition warning.
+ #undef _POSIX_C_SOURCE
+-#undef _POSIX_THREADS
++// #undef _POSIX_THREADS
+ #undef _XOPEN_SOURCE
+ 
+ // Undefine Qt keywords that conflict with Python headers
+-- 
+2.17.1
+

Projects/pythonqt.cmake

@@ -12,3 +12,7 @@ superbuild_add_project(pythonqt
     # platforms.
     -DMACOSX_APP_INSTALL_PREFIX:PATH=<INSTALL_DIR>/Applications
 )
+
+if (UNIX)
+  superbuild_apply_patch(pythonqt fix-native-threads "Disable _PTHREAD unset to get native threads type.")
+endif ()
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