Missing path for pkg-config on amd64 debian based distros
Description of problem:
This error occurs when attempting to configure qemu from git :
```error
ERROR: glib-2.56 gthread-2.0 is required to compile QEMU
```

Although it seems to be as simple as "_just install the dev lib!!!_" it is not that simple.

1. First of all, my system already has the library installed :
   ```sh
   dpkg -l | grep libglib2.0-dev
   ii  libglib2.0-dev:amd64                          2.74.4-1                            amd64        Development files for the GLib library
   ii  libglib2.0-dev-bin                            2.74.4-1                            amd64        Development utilities for the GLib library
   ```
1. Second, the file required by _pkg-config_ does exist aswell :
   ```sh
   ls /usr/lib/x86_64-linux-gnu/pkgconfig/gthread-2.0.pc -l
   -rw-r--r-- 1 root root 240 dez 27 20:42 /usr/lib/x86_64-linux-gnu/pkgconfig/gthread-2.0.pc
   ```
1. Finally, the real problem is that pkg-config is not able to identify it **unless** you specify the _x86-64_ dir :
   - Default usage. It fails.
      ```sh
      pkg-config --modversion gthread-2.0
      Package gthread-2.0 was not found in the pkg-config search path.
      Perhaps you should add the directory containing `gthread-2.0.pc'
      to the PKG_CONFIG_PATH environment variable
      Package 'gthread-2.0', required by 'virtual:world', not found
      ```
   - Fixed usage (temp)
      ```sh
      env PKG_CONFIG_PATH="$PKG_CONFIG_PATH:/usr/lib/x86_64-linux-gnu/pkgconfig/" pkg-config --modversion gthread-2.0
      2.74.4
      ```
Steps to reproduce:
1. clone qemu (master)
2. try to run _configure_
Additional information:
Of course it seems to be a problem related to the program _pkg-config_ itself, or even by the distro's package, but it totally prevents any build of qemu in a debian-based distro, with architecture _amd64_.