build: Always compile the Qt 6 test application with -fpic

* m4/qt6.m4: Do not add -fpic to GPGME_QT6_CFLAGS. Add -fpic to CPPFLAGS
used for build test of simple Qt 6 application.
--

The pkgconfig files of Qt6Core do not contain the qt_config variable,
so that we cannot easily check whether Qt6 was compiled with pic. For
simplicity we always compile the test application with -fpic to avoid
a build failure if Qt6 was actually compiled with pic.
For the actual build of QGpgME libtool automatically uses -fPIC, so that
we don't have to add it to the GPGME_QT6_CFLAGS.

m4/qt6.m4

@@ -23,11 +23,6 @@ AC_DEFUN([FIND_QT6],
                     [have_qt6test_libs="yes"],
                     [have_qt6test_libs="no"])
 
-  if ! test "$have_w32_system" = yes; then
-    if "$PKG_CONFIG" --variable qt_config Qt6Core | grep -q "reduce_relocations"; then
-      GPGME_QT6_CFLAGS="$GPGME_QT6_CFLAGS -fpic"
-    fi
-  fi
   if test "$have_qt6_libs" = "yes"; then
     # Qt6 moved moc to libexec
     qt6libexecdir=$($PKG_CONFIG --variable=libexecdir 'Qt6Core >= 6.4.0')
@@ -64,6 +59,9 @@ AC_DEFUN([FIND_QT6],
     dnl for host.
     OLDCPPFLAGS=$CPPFLAGS
     CPPFLAGS=$GPGME_QT6_CFLAGS
+    if ! test "$have_w32_system" = yes; then
+      CPPFLAGS="$CPPFLAGS -fpic"
+    fi
     OLDLIBS=$LIBS
     LIBS=$GPGME_QT6_LIBS
     AC_LANG_PUSH(C++)