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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<tr><td><span title='2020-10-01 15:47:29 +0200'>2020-10-01</span></td><td><a href='/guix/commit/gnu/tests/virtualization.scm?id=18e76f89055f25f015fadb7c999b410f38a88cc6'>services: hurd-vm: Resurrect system-test by using raw disk-image.</a><span class='msg-avail'>...</span></td><td>Jan (janneke) Nieuwenhuizen</td></tr>
<tr><td><span title='2020-09-29 21:56:28 +0200'>2020-09-29</span></td><td><a href='/guix/commit/gnu/tests/virtualization.scm?id=c11c19bd4d0dc4ec56b949647057dbf00567f2ae'>services: hurd-vm: Add system test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>