Introduce glnx-alloca.h with glnx_strjoina()

This is taken from systemd, and is really useful when one has a few
known-to-be-small strings one wants to concatenate without resorting
to malloc.

Makefile-libglnx.am

@@ -18,6 +18,7 @@
 EXTRA_DIST += $(libglnx_srcpath)/README.md $(libglnx_srcpath)/COPYING
 
 libglnx_la_SOURCES = \
+	$(libglnx_srcpath)/glnx-alloca.h \
 	$(libglnx_srcpath)/glnx-backport-autocleanups.h \
 	$(libglnx_srcpath)/glnx-backport-autoptr.h \
 	$(libglnx_srcpath)/glnx-backports.h \

glnx-alloca.h

@@ -0,0 +1,47 @@
+/* -*- mode: C; c-file-style: "gnu"; indent-tabs-mode: nil; -*-
+ *
+ * Copyright (C) 2014,2015 Colin Walters <walters@verbum.org>.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the
+ * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+ * Boston, MA 02111-1307, USA.
+ */
+
+#pragma once
+
+#include <stdlib.h>
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+/* Taken from https://github.com/systemd/systemd/src/basic/string-util.h
+ * at revision v228-666-gcf6c8c4
+ */
+#define glnx_strjoina(a, ...)                                           \
+        ({                                                              \
+                const char *_appendees_[] = { a, __VA_ARGS__ };         \
+                char *_d_, *_p_;                                        \
+                int _len_ = 0;                                          \
+                unsigned _i_;                                           \
+                for (_i_ = 0; _i_ < G_N_ELEMENTS(_appendees_) && _appendees_[_i_]; _i_++) \
+                        _len_ += strlen(_appendees_[_i_]);              \
+                _p_ = _d_ = alloca(_len_ + 1);                          \
+                for (_i_ = 0; _i_ < G_N_ELEMENTS(_appendees_) && _appendees_[_i_]; _i_++) \
+                        _p_ = stpcpy(_p_, _appendees_[_i_]);            \
+                *_p_ = 0;                                               \
+                _d_;                                                    \
+        })
+
+
+G_END_DECLS

libglnx.h

@@ -24,6 +24,7 @@
 
 G_BEGIN_DECLS
 
+#include <glnx-alloca.h>
 #include <glnx-local-alloc.h>
 #include <glnx-backport-autocleanups.h>
 #include <glnx-backports.h>