Search libraries in GUIX_KF5INIT_LIB_PATH.

Based on an idea by NixOs
pkgs/development/libraries/kde-frameworks/kinit/kinit-libpath.patch

===================================================================
--- kinit-5.32.0/src/kdeinit/kinit.cpp.orig	2017-10-22 21:02:20.908765455 +0200
+++ kinit-5.32.0/src/kdeinit/kinit.cpp	2017-10-22 21:03:25.312818248 +0200
@@ -623,20 +623,18 @@
             if (libpath_relative) {
                 // NB: Because Qt makes the actual dlopen() call, the
                 //     RUNPATH of kdeinit is *not* respected - see
                 //     https://sourceware.org/bugzilla/show_bug.cgi?id=13945
                 //     - so we try hacking it in ourselves
-                QString install_lib_dir = QFile::decodeName(
-                        CMAKE_INSTALL_PREFIX "/" LIB_INSTALL_DIR "/");
-                QString orig_libpath = libpath;
-                libpath = install_lib_dir + libpath;
-                l.setFileName(libpath);
-                if (!l.load()) {
-                    libpath = orig_libpath;
-                    l.setFileName(libpath);
-                    l.load();
-                }
+                // Try to load the library relative to the active profiles.
+                QByteArrayList profiles = qgetenv("KDEINIT5_LIBRARY_PATH").split(':');
+                for (const QByteArray &profile: profiles) {
+                    if (!profile.isEmpty()) {
+                        l.setFileName(QFile::decodeName(profile) + QStringLiteral("/") + libpath);
+                        if (l.load()) break;
+                    }
+		}
             } else {
                 l.load();
             }
             if (!l.isLoaded()) {
                 QString ltdlError(l.errorString());