Reproduction steps
----------------
1. Open "dconf-editor" and set org.gnome.desktop.interface font-name
to "DejaVu Sans Mono Bold 10". (Using gnome-tweak-tool works as well.)
2. Notice that gtk_label_new("any text") is NON-bold.

Expected behavior
----------------
GTK apps respect the users' font-setting, even when it is a bold font.

This is a degrade from GTK+2 to GTK+3. It is easy to see when compiling Wikipedia's "hello world" exampl, 
https://en.wikipedia.org/wiki/GTK%2B#Example

# Correct. Shows bold fonts.
$ cc -Wall helloworld.c -o helloworld $(pkg-config --cflags --libs gtk+-2.0)

# Wrong. Does not show bold fonts.
$ cc -Wall helloworld.c -o helloworld $(pkg-config --cflags --libs gtk+-3.0)

More information
----------------
I first saw this on Fedora 22 and Ubuntu 14.04, but the problem is also in
today's origin/gtk-3-18 (bd3ea2355bee3ff51647131bf2294afd245dd5a) and origin/master (0434649cc14f1e860dca6ff14a2d3ce3aec68ac4).

Workaround (proofs that GTK is indeed requesting the non-bold font):
----------------
Add this to your .fonts.conf

 <match target="pattern">
  <test compare="eq" name="family" qual="any">
   <string>DejaVu Sans Mono</string>
  </test>
  <edit binding="same" mode="prepend" name="weight">
   <const>bold</const>
  </edit>
 </match>