Allow the configuration file and theme directory to be specified at run time.
Patch by Eelco Dolstra, from Nixpkgs.

--- slim-1.3.6/app.cpp	2013-10-02 00:38:05.000000000 +0200
+++ slim-1.3.6/app.cpp	2013-10-15 11:02:55.629263422 +0200
@@ -200,7 +200,9 @@
 
 	/* Read configuration and theme */
 	cfg = new Cfg;
-	cfg->readConf(CFGFILE);
+	char *cfgfile = getenv("SLIM_CFGFILE");
+	if (!cfgfile) cfgfile = CFGFILE;
+	cfg->readConf(cfgfile);
 	string themebase = "";
 	string themefile = "";
 	string themedir = "";
@@ -208,7 +210,9 @@
 	if (testing) {
 		themeName = testtheme;
 	} else {
-		themebase = string(THEMESDIR) + "/";
+		char *themesdir = getenv("SLIM_THEMESDIR");
+		if (!themesdir) themesdir = THEMESDIR;
+		themebase = string(themesdir) + "/";
 		themeName = cfg->getOption("current_theme");
 		string::size_type pos;
 		if ((pos = themeName.find(",")) != string::npos) {
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<div class='path'>path: <a href='/guix/log/?id=ef27d1cd1ee701cd61c76b76299d4138476f9e91'>root</a>/<a href='/guix/log/etc?id=ef27d1cd1ee701cd61c76b76299d4138476f9e91'>etc</a>/<a href='/guix/log/etc/hurd-manifest.scm?id=ef27d1cd1ee701cd61c76b76299d4138476f9e91'>hurd-manifest.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/hurd-manifest.scm?id=ef27d1cd1ee701cd61c76b76299d4138476f9e91&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-08-21 10:45:45 +0200'>2023-08-21</span></td><td><a href='/guix/commit/etc/hurd-manifest.scm?id=3c6b6941a2c76c26ebf0c1bfd7f901a22c19dce1'>maint: Add 'etc/hurd-manifest.scm'.</a><span class='msg-avail'>...</span></td><td>Janneke Nieuwenhuizen</td></tr>