Avoid usage of PATH_MAX.

Taken from https://salsa.debian.org/freedesktop-team/fontconfig/-/blob/master/debian/patches/path_max.patch

Index: fontconfig-2.13.1/src/fccfg.c
===================================================================
--- fontconfig-2.13.1.orig/src/fccfg.c
+++ fontconfig-2.13.1/src/fccfg.c
@@ -2231,7 +2231,7 @@ FcConfigRealFilename (FcConfig		*config,
 
     if (n)
     {
-	FcChar8 buf[PATH_MAX];
+	FcChar8 buf[FC_PATH_MAX];
 	ssize_t len;
 
 	if (sysroot)
>
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<tr><td><span title='2021-07-29 13:09:48 -0400'>2021-07-29</span></td><td><a href='/guix/commit/gnu/system/setuid.scm?id=45235e67e4aeac96ad8d60265bef054990aeb425'>system: Add (gnu system setuid).</a><span class='msg-avail'>...<span class='msg-tooltip'>I flubbed and somehow when including the patch series, this got dropped.
Oops... should fix things again.

* gnu/system/setuid.scm: New file.
</span></span></td><td>Christopher Lemmer Webber</td></tr>