emacs-28 2d647e8: Describe how to debug fontconfig issues

[Robert Pluim]

branch: emacs-28
commit 2d647e88faeb53c15d00e2198da480dbd540a4ec
Author: Robert Pluim <rpluim@gmail.com>
Commit: Robert Pluim <rpluim@gmail.com>

    Describe how to debug fontconfig issues
    
    * etc/PROBLEMS: Add FC_DEBUG usage pointers.
---
 etc/PROBLEMS | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/etc/PROBLEMS b/etc/PROBLEMS
index f396dc3..2d3bc0c 100644
--- a/etc/PROBLEMS
+++ b/etc/PROBLEMS
@@ -751,6 +751,18 @@ Try removing or moving aside 
"$XDG_CONFIG_HOME/fontconfig/conf.d" and
 "$XDG_CONFIG_HOME/fontconfig/fonts.conf"
 ($XDG_CONFIG_HOME is treated as "~/.config" if not set)
 
+Running Emacs as
+
+    FC_DEBUG=1024 emacs
+
+will cause fontconfig to output information about which configuration
+files it is reading.  Running Emacs as
+
+    FC_DEBUG=1 emacs
+
+will result in information about the results of fontconfig's font
+matching (including the filename(s) of the resulting fonts).
+
 *** This can occur when two different versions of FontConfig are used.
 For example, XFree86 4.3.0 has one version and Gnome usually comes
 with a newer version.  Emacs compiled with Gtk+ will then use the