gnu: dbus-c++: Fix build with GCC 7.

* gnu/packages/patches/dbus-c++-gcc-compat.patch,
gnu/packages/patches/dbus-c++-threading-mutex.patch: New files.
* gnu/local.mk (dist_patch_DATA): Adjust accordingly.
* gnu/packages/glib.scm (dbus-c++)[source](patches): New field.
[arguments]: End phases on #t.

gnu/packages/patches/dbus-c++-gcc-compat.patch

@@ -0,0 +1,14 @@
+Fix a string comparison in C++ 11.
+
+Taken from Debian:
+https://sources.debian.org/src/dbus-c++/0.9.0-8.1/debian/patches/06_fix_gcc-7_ftbfs.patch/
+
+--- a/src/pipe.cpp
++++ b/src/pipe.cpp
+@@ -83,5 +83,5 @@ ssize_t Pipe::read(void *buffer, unsigne
+ void Pipe::signal()
+ {
+   // TODO: ignoring return of read/write generates warning; maybe relevant for eventloop work...
+-  ::write(_fd_write, '\0', 1);
++  ::write(_fd_write, "\0", 1);
+ }