[pan] fix: can compile gtk3 without gtk2 libs

[Dominique Dumont <ddumont src gnome org>]

commit d2990c79dddaf1b6fa4606c519761daa67eca597
Author: Dominique Dumont <dod debian org>
Date:   Sun Sep 19 17:48:10 2021 +0200

    fix: can compile gtk3 without gtk2 libs
    
    Closes #116
    
    See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=967894#17
    
    Thanks to Simon for the suggestion

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index ea63c98..efd90f8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -183,7 +183,7 @@ else
                       AC_MSG_RESULT(no)
                       ])
    fi
-   AM_PATH_GTK_2_0($GTK_REQUIRED,,exit 1,gthread)
+   PKG_CHECK_MODULES([GTK], [gtk+-2.0 >= $GTK_REQUIRED, gthread-2.0])
 fi
 
 AC_SUBST(ENCHANT_CFLAGS)