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='2019-03-16 18:15:13 +0100'>2019-03-16</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=f0cc5e7e1e4c03af29c5d4855dc5962502c49147'>booloader: Add 'invoke/quiet'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-12-15 11:52:38 +0100'>2017-12-15</span></td><td><a href='/guix/commit/gnu/build/bootloader.scm?id=e22482038611a53a9d2b25df20363664cd91be2e'>bootloader: Factorize write-file-on-device.</a><span class='msg-avail'>...</span></td><td>Mathieu Othacehe</td></tr>
93497eb2ee3127270dea12be103f1bc2'>maint: Give command to install the pre-push hook.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-03-24 13:56:38 +0100'>2017-03-24</span></td><td><a href='/guix/commit/HACKING?id=ee7c21c7044355fc4c6bbe0906f061f6fbdfb79e'>maint: Mention the Signed-off-by line in 'HACKING'.</a><span class='msg-avail'>...</span></td><td>Clément Lassieur</td></tr>
<tr><td><span title='2017-03-15 15:19:53 +0100'>2017-03-15</span></td><td><a href='/guix/commit/HACKING?id=bc551cf32b6ccb3f8dd60b1d0d4e3c3e88cb1f8d'>maint: Mention guix-patches@gnu.org in 'HACKING'.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-01-30 13:20:13 +0100'>2017-01-30</span></td><td><a href='/guix/commit/HACKING?id=8efc35a8902910ef7c09183c1d79d51498b2c10c'>maint: Fix invalid calls to 'info'.</a><span class='msg-avail'>...</span></td><td>Mathieu Lirzin</td></tr>
<tr><td><span title='2017-01-04 16:27:20 -0500'>2017-01-04</span></td><td><a href='/guix/commit/HACKING?id=69355e1283d300a663fb639ec426f10f3feb404b'>doc: Add a Git hook that verifies signatures before pushing.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>
<tr><td><span title='2016-07-24 19:54:15 +0200'>2016-07-24</span></td><td><a href='/guix/commit/HACKING?id=20e29643449adbb9658c0d3bec9c8b117697fe41'>doc: Explain how to set up Git for signing.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2016-07-14 15:37:51 -0400'>2016-07-14</span></td><td><a href='/guix/commit/HACKING?id=a16db014d10659051037eb76a0424b46e94f2f2d'>doc: Mention commit signatures in HACKING.</a><span class='msg-avail'>...</span></td><td>Leo Famulari</td></tr>