QProcess/FreeBSD: remove the workaround allowing undefined symbols

On FreeBSD, the environ variable doesn't come from libc, but is instead
inserted during linking. See ccf74b5928
(5.6) for more information.

But instead of allowing undefined symbols in QtCore, let's use a weakref
to environ so this one symbol is allowed to be undefined. It won't be,
but the linker doesn't know.

FreeBSD appears to be the only BSD to require this. We used to apply the
same linker option to OpenBSD in Qt 5, but neither the OpenBSD or nor
the NetBSD ports trees[1][2] carry a patch for this, so I don't think
it's necessary.

[1] https://github.com/openbsd/ports/tree/master/x11/qt6/qtbase/patches
[2] https://github.com/NetBSD/pkgsrc/tree/trunk/x11/qt6-qtbase/patches

Pick-to: 6.6
Change-Id: I63b988479db546dabffcfffd17661c839014771a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/CMakeLists.txt

@@ -479,14 +479,6 @@ qt_internal_extend_target(Core CONDITION MSVC AND (TEST_architecture_arch STREQU
         "/BASE:0x67000000"
 )
 
-# QtCore can't be compiled with -Wl,-no-undefined because it uses the
-# "environ" variable and FreeBSD does not include a weak symbol for it
-# in libc.
-qt_internal_extend_target(Core CONDITION FREEBSD
-    LINK_OPTIONS
-      "LINKER:--warn-unresolved-symbols"
-)
-
 qt_internal_extend_target(Core CONDITION QT_FEATURE_xmlstream
     SOURCES
         serialization/qxmlstream.cpp serialization/qxmlstream.h serialization/qxmlstream_p.h

src/corelib/io/qprocess_unix.cpp

@@ -52,6 +52,11 @@
 #  define O_PATH        0
 #endif
 
+#ifdef Q_OS_FREEBSD
+__attribute__((weak))
+#endif
+extern char **environ;
+
 QT_BEGIN_NAMESPACE
 
 using namespace Qt::StringLiterals;
@@ -80,10 +85,6 @@ struct PThreadCancelGuard
 
 #if !defined(Q_OS_DARWIN)
 
-QT_BEGIN_INCLUDE_NAMESPACE
-extern char **environ;
-QT_END_INCLUDE_NAMESPACE
-
 QProcessEnvironment QProcessEnvironment::systemEnvironment()
 {
     QProcessEnvironment env;